ARTHROPODA

ARTHROPODA

792 ANIMAL CLASSIFICATION CHAPTER CXXXVII ARTHROPODA (Jointed-limbed Animals) Until about 1800, these were termed ' i n s e c t s ' in reference t...

1MB Sizes 1 Downloads 88 Views

792

ANIMAL CLASSIFICATION CHAPTER

CXXXVII

ARTHROPODA

(Jointed-limbed Animals) Until about 1800, these were termed ' i n s e c t s ' in reference to their insected (=jointed) bodies and limbs. Cuvier, 1817, gave name A R T I C U L A T A to one of his four main divisions of the animal king­ dom, and divided into four classes : (1) Annelids ; (2) Crustaceans ; (3) Arachnids; (4) Insects. Subsequently annelids were excluded (not without several attempts to bring them back) and the phylum termed A R T H R O P O D A . Centipedes and millipedes were separated from insects (HEXAPODA) and termed MYRIAPODA, and only relatively lately were they separated from one another. Two main streams of arthropods gradually emerged : (1) Gill breathing or branchiate forms, like the crayfish, living in water : (2) Air breathing forms, like the cockroach, living in air. Moseley, a zoologist of the famous ' Challenger' expedition, discovered t h a t the worm-like animal Peripatus, previously classed with annelids, is related to arthropods, and subsequently Sedgwick established (from internal anatomy and development) t h a t it does link the two groups, b u t in the balance of its features it is an arthropod. Fossil evidence accumulated to show arachnids had a marine origin. The existing king-crabs, too, are more akin to arachnids than to crustaceans. Other variations in the classification proposed in recent years are : (1) To include crustaceans, myriapods and insects as M A N D I B U L A T A and to contrast them with arachnids and their kin as CHELICERATA. This system is based on the entirely different mouth parts of the two groups. (2) To treat the genus Peripatus as the type of a separate phylum. (3) To divide the phylum into two or more new phyla, e.g., separating branchiate and tracheate forms, and/or mandibulate and chelicerate. Arthropods are triploblastic, bilateral metazoa, segmented both in body and limbs, the body cavity a haemoccele, the coelom being very reduced, only appearing in the embryo. Hard segmented chitin exoskeleton secreted by epidermis, periodically moulted. Instar s = periods between moults. M e t a m o r p h o s i s often divides life-cycle into adult and larval periods. Segmentation of body (metameric) affects exoskeleton, muscles, nerves, glands. Each segment bears one pair of true segmented a p p e n d a g e s (limbs, etc.) although other out­ growths, not regarded as appendages, such as wing's and gills, may be added. Segments often ± fused in groups (tagmata) in which detailed anatomical study enables us to recognize constituent seg­ ments. T e l s o n = s e g m e n t without limbs, invariably present a t anal end, embryonic in Peripatus and insects, well developed in arachnids, trilobites, crustaceans, centipedes and millipedes. Muscles striped, derived from longitudinal coat. The circular layer, found in annelids, is absent. Some of the anterior appendages modified as jaws or piercing organs, differences important in classification. Mouth ventral, near anterior end, anus terminal, so no tail. E x t e n t of stomodaeum (=fore-gut) and proctodseum (=hind-gut) very great, each lined by thin chitin coat, sometimes thickened in parts of stomodaeum into

ARTHROPODA

793

triturating apparatus. Respiratory organs=skin (rare), gills, lungbooks, or tracheae. Circulatory system of the open or lacunar type. Heart, arteries and veins occur, latter mostly enlarged to form haemocoele, into which arteries open freely. Nervous system and sense organs well developed, essentially the same as in our types. Arthro­ pods seem to be incapable of forming cilia, so ciliated epithelium never occurs,* and even the male gametes are not ciliated, b u t covered by a thin and sometimes spiny coat of chitin. (They differ from those of all other animals except nematodes.) TARDIGRADA (Bear Animalcules) Small forms, less t h a n i mm. in length. Depressed, cylindrical, with rounded ends, no antennae or jaws, four pairs of unjointed legs, body segments ± alike, no special sense organs except eye-spots, muscles non-striated, no circulatory system, two tubes probably representing Malpighian tubules, unpaired gonad opening into intestine, herma­ phrodite. Slow moving, mostly terrestrial on heather, mosses, and lichens, some freshwater and in snow, a few marine, epizoic on algae and holothurians, probably feed on microscopic algae. Resist tempera­ tures ioo° C. to — 270 0 C , also, like nematodes and rotifers, dryness, passing into a dormant condition for indefinite periods. Cosmopolitan. No known fossils. Echiniscus.—ο·2 mm. in length, on moss. Macrobiotus.—0*7 mm. in length, freshwater. ONYCHOPHORA (Peripatoids) Worm-like, creeping on damp soil, in rotten wood, under bark, under logs and stones, some species in termites' nests ; feed on insects, especially termites. Reach 20 cm. in length. One pair of ringed antennae, one pair of oral papillae, one pair of double mandibles, 18 to 44 pairs of similar legs, unjointed, body segments alike, not externally visible, skin being unsegmented, papillate ; tracheae with innumerable openings on surface ; eyes of chaetopod type ; truly segmented ccelom formed in embryo, supplanted by haemoccele ; nephridia paired ccelomoducts ; worm-like unpaired reproductive opening between ultimate or penultimate pair of legs, sexes distinct. About eight living and two fossil genera. Existing forms widely scattered, mostly in southern hemisphere. (Fig. 319.) Peripatus.—Viviparous, cleavage partial, West Indies (except Cuba), South Mexico to Chili, Sumatra. Peripatopsis.—Viviparous, cleavage total, Cape Colony. Peripatoides.—Oviparous or almost so, Australia, New Zealand, New Guinea, New Britain, Malay Peninsula, Siam, North­ eastern India, Central West Africa. * It is alleged to occur in peripatoids, for which reason this group is sometimes excluded from Arthropoda.

794

ANIMAL

CLASSIFICATION

Onychophora, continued.

Ayshearia.—Fossil, Mid-Cambrian, Burgess shale, British Columbia, closely resembles modern form in trunk and limbs (antennae and head not preserved), but marine ! Xenusion.—Fossil, doubtful, from Pre-Cambrian quartzite of Scan­ dinavia.

Fig. 319.—Peripatopsis capensis. A preserved, somewhat shrunken specimen, viewed from dorsal surface.

PENTASTOMIDA = LINGUATULIDA (Tongue-worms) Worm-like, parasitic, reach large size, parasitic in vertebrates, no obvious segmentation internal or external, no antennae, two pairs of hooks near mouth, no limbs in adult, two pairs in larva, short cephalothorax ; no circulatory, respiratory, sensory, or excretory organs ; sexes distinct, sex opening in male just behind mouth, in female near hind end. No known fossils. Linguatula.—Adult female reaches 8 cm. in length, adult male 2 cm., larva 5 mm., eggs extruded, larvae on plants, eaten by rabbits and hares, encyst, eaten by foxes, wolves, dogs and rarely man, adults reaching nasal passages of host. Porocephalus.—Larvae in rodents, adults in rattlesnakes. The last three classes are soft, but the remaining classes (except for a very few specialized members) have a hard covering=well developed exoskeleton.

ARTHROPODA

795

CHILOPODA = OPISTHOGONEATA

{Centipedes) Elongate, depressed, creeping forms, with numerous limbs, carni­ vorous, one pair of antennas, three pairs of jaws (mandibles, first and second maxillae), one pair of limbs on each segment through rest of the body, first pair ( = t o x o g n a t h s ) with poison duct opening near end, others=walking legs, all alike, 15 to 173 segments in all, divided into head and trunk, tracheae, with openings (spiracles) segmental, single or paired ; eyes simple or compound ; sexes distinct, unpaired sex opening on penultimate segment. Devonian to Recent, modern forms from Tertiary (amber of Oligocene) ; modern types usually nocturnal; feed on insects and worms. Some large types devour lizards and mice, creep and burrow in soil and vegetable debris ; indifferent to temperature, same species in hot plains and snow. GEOPHILOMORPHA.—Paired spiracles a t sides, about 40 to over 100 pairs of legs, eyes usually absent. Cosmopolitan. Geophilus.—Several large British species, one phosphorescent. SCOLOPENDROMORPHA.—Paired spiracles a t sides, 21 to 23 pairs of legs, eyes simple. These include the largest centipedes, reaching length of one foot. Cosmopolitan. Scolopendra.—Best developed in tropics. LITHOBIOMORPHA.—Paired spiracles a t sides, 15 pairs of legs with corresponding terga and sterna, some terga reduced; young stage with only 7 pairs of legs ; eyes simple. Cosmo­ politan. Lithobius.—Segments alternately small and large, reaches 2 inches in length in southern Europe. CRATEROSTIGMOMORPHA.—Paired spiracles a t sides, 15 pairs of legs, with corresponding number of sterna, b u t 21 terga ; eyes simple. Australia. Craterostigma.—The only genus. ANARTIOSTIGMA.—Dorsal unpaired spiracles, 15 pairs of legs, with corresponding number of sterna, b u t only 8 terga ; eyes compound. Almost cosmopolitan, but not native to Britain. Scutigera.—The only genus. DIPLOPODA = PROGONEATA (Millipedes) Elongated, with arched terga, creeping forms with numerous limbs, vegetable feeders, one pair of antennae, short and very similar to legs, two pairs of jaws : (1) Mandibles of two or three segments ; (2) Maxillae united to form gnathochilarium. These belong to the head. Then follows thorax, four segments each with pair of append­ ages, finally a b d o m e n of 20 to more than 100 segments, each usually with two pairs of appendages (diplopod) ; respiration by means of tracheae. Sexes distinct, sex openings paired, between second pair of limbs on third segment behind head. Tertiary fossils, existing forms cryptozoic, under stones and logs, or in rotting wood. Many pro­ tected against enemies (insects, birds) by glands opening on terga, producing fluid containing prussic acid. Scanty fossils since Upper Silurian and Lower Carboniferous, slightly more numerous Upper Carboniferous onwards.

796

ANIMAL CLASSIFICATION

Diplopoda, continued,

SYMPHYLA.—A few white forms, reach 6 mm. in length, previously grouped with preceding class, sometimes as a separate class ; connect millipedes and centipedes, and related to primitive insects. Progoneate, sex opening between fourth pair of legs ; most segments not diplopod ; antennae long, unbranched ; two additional pairs of jaws, making four in a l l ; n or 12 pairs of legs, more numerous terga (15); Two genera, both occur in England. Scolopendrella and Scutigerella.—Latter includes so-called garden centipede. PAUROPODA.—Minute forms, only up to 2 mm. in length. Seg­ ments mostly show fusion of terga only, antennae branched into three, ten pairs of limbs, equally spaced, b u t first pair very reduced. Pauropus.—Widespread in Europe, North and South America. PSELAPHOGNATHA.—Body soft, with no definite plates, rows and tufts of bristles, two additional pairs of jaws, eleven seg­ ments in trunk, segments one to four with single pairs of legs, five to eight with double pairs, nine with single pair, and ten and eleven legless. No legs modified as pairing organ. Two genera :— Polyxenus and Lophoproctus.—The former represented in Britain. The preceding are more or less aberrant. Now follow the typical millipedes. In the male the last pair of legs takes the form of pincers, and acts as a pairing organ :— ONISCOMORPHA.—Pill millipedes, short forms capable of rolling into a ball, few segments. Cosmopolitan. Glomeris.—Like a wood-louse, b u t more shiny, British. LIMACOMORPHA.—Slug millipedes, short forms, capable of spiral coiling ; about twenty segments. Two genera. Tropical America and Asia. Glomeridesmus.—Tropical American. HELMINTHOMORPHA.—Worm millipedes, long forms, capable of spiral coiling, many segments. Several subdivisions. Julus.—Often in stems or fruits, common, British. INSECTA = HEXAPODA (Insects) The largest class in number of species, which exceeds t h a t of all other animals, many species exceedingly abundant in number of individuals, in this respect only surpassed by some nematodes and bacteria. Typical ambulatory, cursorial, saltatory and flying forms, with com­ pact bodies differentiated into head, thorax, and a b d o m e n , one pair of antennae, three pairs of jaws, three pairs of walking legs on thorax, the second and third segments of which may bear wings ; no appendages on abdomen unless we include styles on 9 and cerci on 10 ; tracheae opening by paired spiracles a t sides of body, sexes separate ; sex open­ ings near hind end, male on eighth abdominal segment and female on ninth abdominal segment. Insects are classified mainly on the basis of : (1) Their life-cycle ; (2) Their mouth parts ; (3) Their wings, whilst certain orders possess :

ARTHROPODA

797

(4) Special features. The terms used for the first three characters have been explained in dealing with the morphology of this group. There are so many orders of insects t h a t it may be helpful to the student if we divide the group into twelve cycles of affinity :— 1. Primitively wingless (=apterygOte), the young resembling the adults (ametabolous) with biting jaws. These have a wider distribution t h a n other insects. Arctic to Antarctic, and include the earliest insects known as fossils from the Devonian. They include :— PROTURA.—Without antennae, the number of segments increasing during life, the body long and narrow, unknown before 1907, minute and hidden under stones and bark and also occur in peat-bogs, example : Acerentomon. COLLEMBOLA.—Spring-tails, leap by suddenly straightening penultimate segment, a movement not unlike t h a t seen in many crustaceans. Some are vegetable feeders, others on excrement, a numerous group. The largest do not exceed 4 mm. in length. Podura is the best known. THYSANURA.—Bristle-tails, are much larger. They are distin­ guished by the two or three long anal cerci; omnivorous feeders including :— Lepisma.—Silver-fish, domestic pest. Lepismodes.—Yellowish baker's b r a t or fire-brat, a nuisance in bakeries. Both scaly. 2. Winged (pterygote) forms, in which, however, the wings cannotbe flexed, b u t are permanently outstretched. Incomplete metamorphosis, h e m i m e t a b o l o u s , biting. These include :— PALiEOPTERA.—Only known as fossils from Upper Carboniferous and Permian. They grew to an immense size, had little differ­ ence between thorax and abdomen, between first and second pairs of wings, sometimes showing traces of a third, smaller, pair on the first thoracic segment. Stenodictya.—Showed this and very primitive wings. Meganeura.—Had wing span of over two feet ! ODONATA.—Dragon-flies are numerous existing forms a t this stage, among the largest of existing insects, similar wings, no cerci, big eyes, with aquatic carnivorous naiads, and adults devouring insects whilst in flight. Triassic to Recent. Libellula.—Is a large British genus. 3. Winged forms in which the wings can be closed. They have the gradual (paurometabolous) metamorphosis and biting jaws. They include :— ORTHOPTERA.—With fore-wings=leathery wing cases, hind wings thin ; possess anal cerci. Upper Carboniferous to Recent. In the cockroaches the legs are adapted for running, and no sounds produced, b u t odour. Blatta orientalis.—Arrived in this country in 16th century and now established everywhere, females with reduced wings. Periplaneta americana.—Name indicates origin, especially in docks and seaport warehouses, both sexes fully winged, deeper brown, two pale bands on first thoracic segment.

798

ANIMAL CLASSIFICATION

Insecta—Orthoptera, continued.

Phyllodromia germanica.—Arrived from Crimean War, smaller, both sexes winged, yellowish-brown, two dark brown bands on first thoracic segments. In the grasshoppers, etc., the hind legs are adapted for jumping and, especially in males, produce sounds :— Gryllus domesticus.—House cricket. Gryllotalpa vulgaris.—Mole cricket, adapted for burrowing. Chorthippus bicolor.—Short horned grasshopper. Tettigonia viridissima.—Great green grasshopper, long-horned. All the above are British, with relatives. The group also includes :— Mantis religiosa.—The praying mantis. Dixippus morosus.—Stick insect, Southern Europe. Phyilium scythe.—Leaf insect, India. DERMAPTERA.—Earwigs are sometimes separated as an order. Their cerci are strong forceps. They have hard wing covers and their fan-like wings are unlike those of other insects. Forficula auricularia.—Is the common British species. ISOPTERA.—Termites or White Ants. Social insects with the caste system. Wings alike when present, short or absent in some castes, sexual forms pigmented, others pale. The wings break off in winged forms. They feed on wood by means of symbiotic flagellates. Tropical and subtropical, including Southern Europe, Tertiary fossils (some from Northern Europe). Can live only in damp, and construct underground channels and large nests. Eutermes.—Australian, nests reach 20 feet in height. EMBIOPTERA and ZORAPTERA.—Are small groups sometimes thought to be related to the termites. The latter also has some affinity with bird-lice (q.v.). Sub-social. 4. Closed-winged forms, with a tendency towards reduction of wings and the adoption of a more or less ectoparasitic life on plants and animals, usually with pricking jaws and gradual (paurometabolous) development. Bugs and lice, a rather hetero­ geneous assemblage in which we can distinguish the following orders :— HOMOPTERA.—Aphids and Scale Insects. Pricking jaws, wings when flexed giving roof-like appearance, uniform, two pairs, sometimes reduced to one or none. Ectoparasites, creeping or fixed, mostly on plants. Cicada.—Largest, many years development, males kept in cages in classical times for chirping song. Aphrophora.—' Cuckoo-spit ' = masses of froth as covering for nymphs. Aphis.—Green-fly, species on many plants. HEMIPTERA.—Bugs. Pricking jaws, body and flexed wings flat, fore wings proximal half ± leathery, distal half membranous (hence name of group), hind pair wholly membranous. Hydrometra.—Pond-skaters. Nepa.—Water scorpion. Notonecta.—Back-swimmer. Corixa.—Water boatman, wingless.

ARTHROPODA

799

Cimex.—Bed-bug, described elsewhere (see p . 528). ANOPLURA.—True lice. Pricking jaws, no wings, small, parasitic on mammals or birds. Pediculus.—Human lice, described elsewhere. THYSANOPTERA.—Thrips. Pricking jaws, wings=narrow bars, fringed on both sides with hairs (i.e., like small feathers). Small, few exceed 1 mm. in length, parasitic on plants. Heliothrips.—Black-fly of greenhouses. PSOCOPTERA.—Paper and Bark mites and Book lice. Modified biting jaws ; maxillary pick makes tapping noise, fore-wings large, roofed a t rest, hind wings small, slightly folded at base at rest, some species wingless. Troctes.—Book-louse, white, wingless. MALLOPHAGA.—Bird lice. Biting jaws. No wings. Minute flat bodies. Devour skin, feathers or hair. Parasitic, creeping, on birds or mammals. Menopon.—On chickens. 5. Winged forms in which the wings can be closed, h e m i m e t a b o l o u s . The larvae are aquatic, have biting jaws, b u t in the adult the jaws are useless and the insect does not feed. External tracheal gills. PLECOPTERA.—Stone-flies. Hold their filmy wings horizontal, flat a t rest. The fore-wings are narrow, the hind broad and pleated. Two long anal cerci. Naiads in water. Perla.—Frequents running water. British. EPHEMEROPTERA.—May-flies. Hold their filmy wings vertical at rest. Fore-wings large, hind wings small, neither pleated. Three long anal cerci. Naiads in water, except last instar (sub-imago). Imago survives only one or two days. Ephemera.—Common, British. Under the following headings 6-12 we include h o l o m e t a b o l o u s insects with larva, pupa and i m a g o stages. The pupa is usually a resting stage, separating the preceding and very different larva from the succeeding and much modified adult. Mostly Triassic to Recent. 6. Linnaeus placed together all transparent winged insects with numerous veins, b u t the dragon-flies, stone-flies, may-flies and termites, having a different development, have been removed. N E U R O P T E R A . — W a s the name given to this group. I t is now confined to forms with roofed wings, most with campodeiform larvae (so called from its resemblance to Campodea, one of the bristle tails) and the pupa usually has a cocoon. In three genera the pupa crawls just before metamorphosis. Larvae generally feed on rotten wood or are carnivorous. Biting mouths (but ant-lions suck). Myrmeleon.—Ant-lion (see p . 560). Sialis.—Alder-fly, eggs (clusters) and larvae in quiet streams. MECOPTERA.—Scorpion-flies. Roofed wings biting mouths, b u t eruciform (i.e., caterpillar-like) larvae, often with pro-legs on all segments of abdomen, pupa actively moving, an unusual feature. Males with turned up ends, like scorpion. Panorpa.—Common scorpion-fly, early stages in soil.

800

ANIMAL CLASSIFICATION

Insecta, continued.

7. TRICHOPTERA.—Caddis-flies. Scientific name from hairy character of front wings. Roofed. Hind wings pleated a t rest. Licking mouths. Larvae known as ' caddis-worms' aquatic, with fully developed thoracic appendages ( c a m p o deiform), spin silken threads to form tube around abdomen, incorporating foreign bodies, space for external gills inside tube. Animal pegged in tube by outgrowths on first abdominal segment and hooks at hind end. Pupa in case. Phryganea.—One of several common genera. 8. LEPIDOPTERA.—Butterflies and Moths. Wings large, scaly. Fore and hind wings not very different. Sipping mouths, maxillae modified to form proboscis, coiled when not in use. Adults chiefly living on nectar, important as flower pollinators. Larvae mostly on vegetation (clothes-moth an exception). Larvae eruciform, usually with jointed legs on thoracic segments (2, 3, 4) and pro-legs on 7, 8, 9, 10 and last. Geometrid-larvae, called ' loopers', have only two pairs of pro-legs, on 10 and last segment. They arch middle legless part of body. Pupae with cocoon, wings and legs soldered to body (obtected). Often their hard­ ened coat gleams with a golden lustre, hence the term chrysalis (plural: chrysalides). Two divisions are conveniently recog­ nized :— RHOPALOCERA.—Butterflies. Club-shaped ending to antennae, wings horizontal when at rest, diurnal. Pieris.—Cabbage-white, larva green, with yellow stripes and dark spots, on crucifers and nasturtium. HETEROCERA.—Moths. No club-shaped ending to antennae, wings vertical a t rest, nocturnal. Tinea.—Clothes-moth. 9. DIPTERA.—True flies, including midges, gnats, mosquitoes and crane-flies. Single pair of wings on middle segment of thorax. Hind pair only represented by pair of drum-stick-like structures (halteres) serving only as balancers. Mouth pricking or sipping. Larvae=maggots, legless (vermiform larvae ; v e r m i larves) often with biting mouths, and may be unsegmented, like those of bots, or segmented as in the crane fly (' leather jackets'). They are sometimes even parasitic, as in bots. Pupa retained in last larval skin, which acts as a cocoon (pupariuxn), often hard and brown, with no impressions of limbs or wings (coarctate). Musca.—House-fly, halteres covered by special flaps, eggs and larvae in refuse and excrement. Calliphora.—Blow-fly or blue-bottle, eggs and larvae in carcasses. Tipula.—Crane fly or daddy-long-legs, larvae=leather jackets. Glossina.—Tsetse fly, similar in appearance to house-fly, b u t blood-sucking. Syrphus.—Hover-fly, eggs on plants infested with aphids, which larvae eat, Sucking them dry and rejecting skin. Adults hover and visit flowers. Culex, Anopheles.—Gnats or mosquitoes, with aquatic larvae.

ARTHROPODA

801

io. SIPHONAPTERA.—Fleas. No wings, small, laterally com­ pressed bodies, jump. Pricking mouths. Larvae vermiform, live in dirt. Pupa in spun cocoon with legs and wings free (exarate). Adults blood-suckers. Pulex.—Common flea. i i . COLEOPTERA.—Beetles and weevils. Largest order of insects. Anterior wings hard and veinless ( = e l y t r a ) , posterior membranous, folded. Head embedded in very large prothorax, giving characteristic appearance. Larvae or grubs very longlived in some, c a m p o d e i f o r m as in ground-beetles and waterbeetles, scarabeiform (round, with short legs) as in scarabs, or v e r m i f o r m (legless) as the meal-worms.* Pupa with wings and legs free (exarate). Coccinella.—Lady-bird, adults and especially larvae feed on aphids, pupa attached to a leaf, antennae clubbed. Leptinotarsa.—Colorado beetle, damages potato crops. Calandra.—Grain weevil, damages stored grain and meal. The weevils have head prolonged forward, with mouth in front. Melolontha.—Cockchafer, adult destroys foliage of trees, larvae roots of crops. Antennae of adults club-shaped and composed of flattened plates (lamellicorn). Agrtotes.—Click-beetle or skip-jack, adults in flowers, larvae (wireworms) destroy roots of plants. Staphylinus.—Rove-beetle, very short wing cases, end of abdomen can be raised, carnivorous. Gyrinus.—Whirligig beetles, Dytiscus—Carnivorous water beetle, and Hydrophilus—Vegetarian water beetle, are aquatic. STREPSIPTERA.—Are minute forms, possibly derived from beetles by reduction. The male has reduced elytra, voluminous hind wings, and the female is wingless. The larvae and adult females are parasitic in bugs, wasps, and bees. 12. HYMENOPTERA.—Saw-flies, Ichneumon-flies, Wasps, Bees, and Ants. Both pairs of wings transparent, membranous, with few veins. Licking mouth. Ovipositor a t hind end in female, modified to s t i n g in higher forms. Waist usually present, between first and second abdominal segments. Larvae and pupae absolutely helpless, usually v e r m i f o r m , occasionally naupliform (resembling Nauplius larva of crustaceans, as in Platygaster). Larvae of lower forms, e.g., saw-flies, vegetable feeders, more advanced types provided with food (parasitoids), sometimes, in the form of pre-masticated insects, the adult receiving secretion in return (trophallaxis). In still higher types honey, pollen, and jelly are provided. The following is a rough, and not strictly scientific, classification of this large group :— SESSILIVENTRES.—Saw-flies. No waist, ovipositors, larvae plantfeeding ± eruciform, non-social. THchosoma.—Saw-fly. Sirex.—Horn-tail, large. * Larvae of certain beetles of the family Lycidae bear a remarkable resemblance to the fossil trilobites. B

26

802

ANIMAL

CLASSIFICATION

Insecta, continued.

PARASITICA.—Ichneumon-flies. Waists, ovipositors. Larvae live on eggs or larvae of other insects provided for them, vermiform, non-social. Rhyssa.—Largest British representative, ovipositor longer than body. VESPOIDEA.—Wasps. Waist, sting, larvae live in cells of mud, rotten wood, or wasp-paper constructed for them, and are fed on larvae or adults of other insects ; sub-social or social. Vespa.—Social wasp. APOIDEA.—Bees. Waist, sting, branched hairs and pollen baskets (corbicula) on legs, secrete wax (in glands between abdominal somites). Larvae live in cells of mud, fibres, leaves, or wax, fed on mixture of nectar and pollen. Later fertile types get regurgitated jelly (royal-jelly) ; sub-social or social. Apis.—Hive-bee. (See pp. 581-582.) FORMICOIDEA.—Ants. Two or more waists, elbowed antennae, detachable wings in female. Larvae fed on regurgitated food, and imago liberated from pupa by efforts of other adults. All social. Dory Una.—Driver ants, carnivorous, tropical, travel in large droves, no nests. —' Formica.—Includes largest British species, vegetarian, in ant­ hills. CRUSTACEA (Crustaceans) Third largest class in number of species. Aquatic, mostly marine, ambulatory and swimming forms. A few have invaded the land. Body differentiated into head and trunk, the latter sometimes showing distinct thorax and a b d o m e n . A distinct telson a t the hind end, two pairs of feelers (antennules, antennae), three pairs of jaws (mandibles, m a x i l l u l e s , maxillae) on head, appendages on trunk variously differentiated, gills or branchiae as breathing organs, often protected under a gill-cover ; sexes usually distinct, genital opening near middle of body, on under surface. Larva, Nauplius, in primitive forms. Cambrian to Recent. The first seven divisions (1-7) are called E n t o m o s t r a c a , but they are a miscellaneous assemblage. The last five (8-12) are called M a l a c o straca and have twenty segments (with some exceptions) as in the crayfish, and like t h a t animal have distinct thorax and abdomen, the former bearing parieopods, the latter pleopods. Stalked eyes often occur in these groups, and gastric teeth. Nauplius stage often omitted. OSTRACODA.—Microscopic or almost so, abundant in marine and fresh water. Bivalved hinged carapace enclosing head, trunk and limbs. Small compound sessile eyes, mandibular palp usually forked, not more than two pairs of leg-like appendages (stenopods) not forked. Scavengers living on dead animals, swimming with both pairs of antennae. Parthenogenesis common in some species, males unknown. Nauplius larva bivalved.

ARTHROPODA

803

Cypris.—Simple ovoid shell, planktonic. NOTOSTRACA.—Tadpole shrimps. Of medium size. Long bodied with numerous pairs of swimming trunk appendages, leaf-like (phyllopods) forked, carapace large flat, eyes com­ pound, sessile. Feed on detritus. Nauplius larva. Apus.—Reaches 3 cm. in length, British, not common. ANOSTRACA.—Fairy shrimps and Brine shrimps. Of medium size. Long bodied with numerous pairs of swimming trunk appendages, leaf-like, forked, no carapace, eyes compound, stalked b u t immovable, sometimes two pairs of eyes. Swim on back. Nauplius larva. Artemia.—Brine shrimp, reaches 10 mm. in length. Chirocephalus.—Fairy shrimp, reaches 25 mm. in length. CONCHOSTRACA.—Clam shrimps. Of medium size. Fresh­ water. Relatively short bodied with numerous pairs of swim­ ming forked leaf-like trunk appendages. Bivalved carapace, hinged, marked by growth-lines (distinction from ostracods), completely covers eggs attached to appendages; sessile com­ pound eyes. Detritus feeders, swim near bottom by jerks of large antennae. Nauplius larva, usually. Estheria.—Reaches 20 mm. in length, American and European, not British (none in our fauna). CLADOCERA.—Water-fleas. Microscopic mostly T J ^ in. to ^ in., chiefly in freshwater. Relatively short-bodied with five, sometimes six, pairs of forked, leaf-like trunk appendages. Bivalved carapace, not hinged and which does not enclose head, b u t forms a b r o o d - p o u c h for the eggs in the female, vestigial in smaller male. Swim in vertical position by jerks of antennae. No larva usually. Plankton. Feed on plankton algae, especially diatoms. Daphnia.—Spine a t posterior end. Simocephalus.—No spines. Both common in freshwater. COPEPODA.—Copepods. Typical members are very abundant, almost microscopic, constituents of plankton, furnishing a large part of the food for fishes and whalebone whales, themselves feeding on plant plankton. Certain types, some larger t h a n free forms, are parasitic. Relatively short bodies. No cara­ pace. Trunk appendages leg-like (stenopods), usually five pairs, the first and sometimes the sixth unforked, the others forked. Median unpaired simple (cyclopean) eye in some. In others paired compound eyes added, b u t median eye persists in vestigial condition ; all eyes sessile. Sexes distinct, some­ times different. Eggs in typical copepods carried by female in a pair of sacs formed of a hardened glutinous secretion. Nauplius larva in most. Cyclops.—Freshwater, almost all ponds, pear-shaped, tapers behind. Calanus.—Marine, very abundant, reaches 4 mm. in length. One species forms main food for herring and mackerel. Lernea.—Fish louse, worm-like, limbs reduced, parasitic on fresh­ water fishes, egg sacs typical.

804

ANIMAL CLASSIFICATION

Crustacea—Copepoda, continued.

Argulus.—Fish louse, discoid, first pair of legs form sucking disks, compound eyes, parasitic on freshwater and marine fishes, b u t eggs laid on stones. CIRRIPEDIA.—Barnacles. Attached, sessile or stalked forms, fixed to substratum a t rostrum by cement developed in glands embedded in antennules. Mouth swings round to open in opposite direction with mantle cavity formed b y carapace around most of head and whole of trunk and appendages. Latter = six pairs used for food intake. Some much modified parasites. Usually hermaphrodite, sometimes with additional, smaller (complemental) males. Eggs develop in mantle cavity, set free as Nauplius larvae, which become Cypris larvae (resembling t h a t genus) and finally settle down as adults. All marine. Originally classed as molluscs, the barnacles were shown to be crustaceans in 1830 by Vaughan Thompson from development. Balanus.—Acorn barnacle, sessile on rocks by wide base, very common. Lepas.—Goose barnacle, stalked, carapace with five separate plates, on driftwood, formerly thought to grow on trees and to produce the barnacle goose! Both the preceding cause the fouling of ships. Sacculina.—Sac-like parasite, without plates, sending branched haustoria through body of crab, destroying sex organs of latter. PHYLLOCARIDA.—A few forms intermediate between entomostracans and malacostracans. Head, thorax, and abdomen as in the latter, but abdomen has additional segment, limbs forked, leaf-like, little differentiated. Bivalve carapace en­ closing head, thorax, and first four abdominal segments, eyes stalked, two pairs of excretory organs, opening on second antennas and second maxillae. (The latter alone occurs in ostracods, the former alone in all other Crustacea.) Eggs carried by setae on thoracic limbs, no larva. Small marine forms feeding on strained particles. Nebalia.—Littoral, under stones, reaches 12 mm. in length. SYNCARIDA.—Mountain shrimps. A few forms intermediate in structure between the next two groups, and more generalized. Head, thorax, and abdomen with usual number of segments found in malacostracans, first thoracic segment usually fused with head ; no carapace, but sometimes stalked eyes. Append­ ages forked. Freshwater, egg sometimes distributed by birds. Anaspides.—Alpine pools, Tasmania. Bathynella.—Subterranean waters, Europe, including England. PERACARIDA.—Malacostracans with no carapace, or one poorly developed, i.e., not uniting with more than four thoracic seg­ ments. Eyes usually sessile. Eggs borne in pouch formed from plates ( = oostegites) on thoracic limbs. An immense group, which must be divided (A-F) :— A. MYSIDACEA.—Opossum shrimps. Retain the primitive malacostracan features (caridoid facies). Carapace extends over

ARTHROPODA

805

most of thorax, b u t fuses only with first three thoracic segments. Eyes on movable stalks. Uropods lamellar. Marine with luminous organs, many bathybic or abyssal or even freshwater. Mysis.—In lakes. B. CUMACEA.—Caridoid. Carapace fused over 3rd or 4th thoracic segments. Eyes sessile, usually united. Uropods styliform. Marine. Diastylis.—Burrows, reaches 10 mm. in length. C. TANAIDACEA.—Modified caridoid. Carapace fused over i s t or 2nd thoracic segments. Eyes on short immovable stalks or absent. Uropods reduced, narrow. Tanais.—Burrows, lines burrow with secreted fibres. D. ISOPODA.—Isopods or Sow-bugs. Mostly depressed forms, flattened from above downwards. No carapace, b u t first thoracic segment fused with head. Eyes sessile or on im­ movable stalks. Uropods usually not distinct. A very large group ; includes marine forms from great depths to shore-line, freshwater and land forms, some of the latter with tracheae as in insects. Three different parasitic series, with innumerable and some much modified representatives. One deep water form reaches a foot in length but most are less than one inch. Idotea.—Marine, British, common. Limnoria.—Gribble, marine, wood-boring, damages piers and wharves. Asellus.—Ponds, ditches. Ligia.—Sea-slater, rock-pools, littoral. Oniscus.—Wood-louse, brown, in gardens. Porcellio.—Grey, in gardens. Armadillium.—Pill-bug, can roll itself into a ball especially well. Bopyrus.—Much modified parasite in gill chamber of prawns, with dwarf males. E. AMPHIPODA.—Amphipods or Scuds. Mostly c o m p r e s s e d , i.e., flattened from side to side. No carapace. Eyes sessile. Uropods styliform. The typical amphipods (sandhoppers) are intertidal, some forms also occurring on the damp soil of tropical forests and on arctic shores, where they are large and abundant. Talitrus.—Common sandhopper, on sandy beaches. Orchestia.—Shorehopper, among rocks. Chelura.—Bores marine timber. Gammarus.—Common, one species Gammarus pulex well known in freshwater. Less typical examples include :— Phoronima.—With very large divided eyes, pelagic, planktonic, transparent, living inside transparent molluscs, salps, pyrosomas and jelly-fishes. Reaches three inches, sometimes five inches in length. Caprella.—Skeleton-shrimp, body cylindrical; very narrow, loop like geometrids over zoophytes and seaweeds, swims by wriggling movements, like mosquito larvae. Cyamus.—Whale-louse. Body broad and flat, cling, often in large numbers, to skin of whales, gnaw pits in skin, no motile stage, transfer by host contact.

806

ANIMAL

CLASSIFICATION

Crustacea—Peracarida, continued. F. THERMOSBiENACEA.—Amphipods specialized for life in hot springs. Monotypic. Thermosbcsna.—In water a t 48 0 C , Tunis. EUCARIDA.—Malacostracans with carapace fused with all the thoracic segments, giving the typical cephalothorax as described in the crayfish. Stalked movable eyes. No brood-pouch. Four divisions (A-D) :— A. EUPHAUSIACEA.—(Formerly placed with mysids as ' Schizopoda '.) Caridoid. Thoracic as well as abdominal appendages forked, no special maxillipedes. Phosphorescent organs. Nauplius and Zocea larvae, mostly micro-feeders (algae). Euphausia.—Krill or kril, chief food of some whales, occur in huge shoals in surface waters of Arctic and Antarctic. DEC APOD A.—B, C, and D can be classed as Decapoda. The first three pairs of thoracic appendages are forked m a x i l l i p e d e s , the five remaining pairs being unforked and generally large. Five main larval forms (often suppressed) may occur: (1) Nauplius—first three pairs of limbs ; (2) Metanauplius—the same + next four as rudiments ; (3) Protozooea—the same + rudimentary abdomen ; (4) Zocea—abdominal appendages, un­ forked ; (5) My sis—abdominal and thoracic appendages forked (schizopod). Naturally only the first four occur in A, as the adult corresponds with (5). B. MACRURA.—Abdomen large, symmetrical, with tail-fan. Penceus.—Commonest prawns of tropical and subtropical seas. First three pairs of legs chelate, all five larval stages. In the following the larval history is more or less abbreviated. Palcemon.—Common prawn, first two pairs of legs chelate. Pandalus.—Pink shrimp. Similar to common prawn b u t smaller. Hippolyte.—iEsop prawn, humped back, changes colour. Crangon.—Shrimp, first pair of legs sub-chelate. Homarus.—Lobster, first legs=huge chelate claws, asymmetric. Nephrops.—Norway lobster, more slender claws, deep waters of Norway and Iceland. Palinurus.—Crawfish, spiny, larger than lobster, b u t much smaller claws, large flat transparent larva, Phyllosoma peculiar, but schizopod and apparently corresponds with (5). Astacus.—Crayfish, freshwater. No larval stages. C. BRACHYURA.—Abdomen very small, bent beneath thorax, symmetrical, no tail-fan, as last pair of abdominal appendages ( = uropods) are absent. There is no schizopod larva. There is often a Zocea, b u t this is succeeded by a Megalopa, which is something like an adult crayfish. Cancer.—Edible or great crab, adults in deep water, juvenile forms in rock-pools, carapace wider than long, oval. Carcinus.—Shore-crab, between tide levels, carapace not quite so wide as in preceding, last pair of legs end in plate for swimming. Other crab families distinguished by different shaped thorax (round, triangular, square). D. ANOMURA.—Abdomen intermediate in size, may be bent under thorax, always ± spirally coiled, abdominal appendages

ARTHROPODA

807

fully developed on one side only, last pair of thoracic reduced and last abdominal appendages much reduced, small tail-fan. Pagurus.—Hermit crab, abdomen in discarded gastropod shell. Galathea.—Squat-lobster. Porcellana.—Porcelain-crab, small. HOPLOCARIDA.—Mantis Shrimps. Malacostracans with cara­ pace leaving last four thoracic segments uncovered. Stalked movable eyes. No brood pouch. Large abdomen. Ocular and antennular segments of head movable. Large claws=2nd pair of thoracic limbs, have clasp-knife action. Reach one foot in length in tropics. Mostly in warmer seas. Burrow in sand or get into crevices in rock and await prey. Many transparent. Eggs in mouth. Larval forms of several kinds. . Squilla.—British, reaches three inches in length. ARACHNIDA (Arachnids) Second largest class in number of species. Body consists of: (i) Cephalothorax or p r o s o m a of a t least six segments bearing two pairs of what are regarded as head appendages and usually four pairs of legs ; (2) Abdomen or o p i s t h o s o m a of twelve segments, sometimes difierentiated into a m e s o s o m a of seven segments and a znetasozna of five, none of which bears appendages (except modified ones in scorpions) ; (3) Telson. Often a waist or peduncle between cephalo­ thorax and abdomen. The head appendages are : (1) Chelicerse, two-jointed jaws, opening like a pocket-knife ; (2) Pedipalps, like small feet, b u t used as feelers. There are no antennae. Eyes simple, in spiders often four pairs. Air-breathing land or aerial (but not flying) animals, respiring by means of l u n g - b o o k s or tracheae, or both, situated on segments 3, 4, 5, and 6 of the mesosoma. Gillbreathing relatives are placed separately. Silurian to Recent. SCORPIONIDEA.—Scorpions. Elongate, large, cephalothorax com­ pact, no waist, mesosoma broad, metasoma narrow and up­ turned, telson a bottle-shaped sting. Genital o p e r c u l u m = plate in mid-line covering genital pore on first mesosomatic somite, and pectines, a pair of combs on second mesosomatic somites, represent modified abdominal appendages. Lungbooks. In hot countries, burrowing or hiding by day, ambula­ tory, carnivorous a t night, never drink, viviparous, young carried on backs of female, adults solitary unless mating. Euscorpio.—Southern Europe. P E D I P A L P I . — W h i p scorpions. Elongate, many have deep waists, mesosoma wide, metasoma narrow, telson a whip-like flagelluzn, or absent. First pair of legs also terminate in flagella. Lungbooks, two pairs. In hot countries, hiding beneath stones and logs by day, or burrowing. Carnivorous, killing prey by im­ paling on spines of palps. Oviparous, mucilage covering of eggs sticks them to underside of abdomen of female. Thelyphonus.—Large claw-like palps. PALPIGRADI.—Micro Whip scorpions. Elongate, minute. Waist. Abdominal segments only ten. Mesosoma wide ; metasoma narrow, only three segments. Telson—flagellum. No distinct

808

ANIMAL

CLASSIFICATION

Arachnida—Palpigradi, continued.

terga and sterna, no respiratory system, no eyes. Under stones in moist places. Kcenenia.—Only genus, several species, widespread. ARANEIDA.—Spiders. Short, mostly moderate-sized to small. Waist. Abdominal segments rarely distinct, no differentiation of meso- and metasoma. Lung-books, two pairs or one, in latter more common case also one pair of tracheae. Poisonducts opening a t tips of chelicerae. S p i n n e r e t s = p a i r e d finger-like processes on abdomen near end, and producing g o s s a m e r , primarily used : (i) For spinning a cocoon to enclose eggs, deposited in burrow, hidden, or carried by female ; (2) For flying threads ; (3) In some species for making w e b s for snaring the prey. Lycosa.—Wolf-spider, carries cocoon and young. Lies in watch and hunts prey. Thomisia.—Crab-spider, moves sideways, lurks on bark, leaves or in flowers. Tegenaria.—House-spider, sheet-like horizontal snares (cobwebs) with chamber beneath, wherein spider lurks. Argyroneta.—Water-spider, constructs dome-shaped huts below pond water and stocks with air, carried down bubble by bubble. Epeira.—Garden-spider, constructs typical webs. Mygale.—Two pairs of lung-books, no tracheae, jaws work vertically (most spiders horizontally). Construct gossamer tubes below or partly below ground, sometimes with movable lid (trap-door spiders). SOLPUGIDA=SOLIFUGiE.—Vinegarones. Large, somewhat elon­ gate. Head of three segments separated from thorax of three segments. Abdomen of ten segments also separate, b u t not divided into meso- and metasoma. Opposite legs meet, so no sterna. Respire by two or rarely three pairs of tracheae. Warm dry tropical, feed on insects, lizards, and mice by day and night, rapid, powerful, and aggressive, oviparous, do not spin. Galeodes.—Largest, reaches 12 cm. in length. PSEUDOSCORPIONES=CHELONETHIDA.—False or Book scor­ pions. Rather small (1 to 6 mm.). Compact cephalothorax. No waist. Abdomen not differentiated into meso- and metasoma. Opposite legs meet, so no sterna. Two pairs of tracheae. Secrete silk-like substance from cephalothorax, manipulate it by comb­ like organs (serrula) on chelicerae, building nests for hibernation and moulting, and cocoons attached to mother ; food passed from mother's glands first to eggs then to young, which suck! After second moult, mother bites hole in cocoon and young escape. Among seaweed, moss, under stones, sometimes among books in libraries. Feed on mites. Sometimes epizoic (not parasitic) on large insects. Chelifer.—Carapace narrow in front. Obisium.—Carapace broad in front. P H ALANGID A = O P I L I O N E S . — H a r v e s t m e n . Medium sized, short, ovoid body. No waist. Abdomen not differentiated into meso- and metasoma. No movement between segments which,

ARTHROPODA

809

however, are well marked. Sterna narrow. Surface sculptured. One pair of tracheae. Stink-glands opening on cephalothorax. Legs usually long. Nocturnal, omnivorous, tearing food with chelicerae, no poison, no silky secretion, do not build. Eggs laid underground, not in cocoon, no maternal care. Phalangium.—British, common in late summer and autumn. PODOGONA=RICINULEL—Small, shortish bodies, waist between first and second abdominal segments, concealed between cephalo­ thorax and third abdominal segment. Third, fourth, fifth, and sixth large. Seventh, eighth and ninth small, concealed in sixth. Movable plate (cucullus) on front of carapace conceals chelicerae. No eyes. Second, third, and fourth pairs of limbs meet in mid-line. Fourth can lock with third abdominal somite, thus immovably uniting cephalothorax and abdomen. Single pair of tufted tracheae. Tropical forests of West Africa and Brazil, burying in soil. Cryptostemma and Cryptocellus.—Only two existing genera. ACARINA.—Mites and Ticks. Minute, sometimes microscopic, with compact, usually ovoid, body, no obvious segmentation, no waist, cephalothorax and abdomen fused, b u t a small portion of the head region, as a base for mouth-parts, separated from the rest as capitulum. The eyes, when present, do not occur on this. Legs widely separated on the two sides, the first two pairs also separated in front from the two hinder pairs. Respira­ tion by tracheae, one pair, sometimes four pairs, sometimes only b y skin. Six-legged larva (Leptus) often distinguishable. Sometimes also a n y m p h = p u p a l stage. Scavengers, ecto­ parasites on plants and animals sometimes sucking blood, a few endoparasites. Usually less than i mm. in length, but parasitic ticks often reach size of pea. Tetrarhynchus.—Red spider, sucks leaves, causing blisters. Female spins web. Tyroglyphus.—Cheese-mite. Tarsonemus.—In tracheae of honey-bee, Isle of Wight disease. Sarcoptes.—Itch-mite ; after pairing female burrows in skin of mammals, swells, dies in burrow. Eriophyes.—Only two pairs of legs ; forms galls on plants. EURYPTERIDA=GIGANTOSTRACA (Water Scorpions) Known only as fossils, shown in last century through work of Hugh Miller on Old Red Sandstone (Devonian). Large scorpion-like forms, body consisting of : (i) Cephalothorax of six segments bearing pair of chelicerae, four pairs of short walking legs (the first pair homologous with pedipalps but not differentiated) and lastly, a pair of oar-like flippers ; (2) Mesosoma of six wide segments bearing gills ; (3) Metasoma of six gradually narrowing segments; (4) A spine-like telson. Two kidney-shaped compound eyes, widely spaced, two simple median eyes nearer mid-line. Reduced abdominal appendages include genital opercula differing in the sexes. Large shallow waters and lagoons, possibly brackish. Cambrian to Carboniferous, b u t best developed in Silurian and Devonian. The largest reached nine feet in length. Hughmilleria and Eurypterus.—Less than one foot.

810

ANIMAL

CLASSIFICATION

XIPHOSURA (King Crabs) Mainly known as fossils (five extant species in three genera). Wide bodied, convex above, spine behind. Consists of : (i) The very wide cephalothorax, semicircular in front, and bearing little differentiated chelicerae, five pairs of walking legs, the first homologous with pedipalps ; the bases of the legs serve as jaws ; (2) The gradually narrowing abdomen, with paired appendages, the first pair bearing the genital operculum, the rest bearing g i l l - b o o k s for respiration. The existing forms show six abdominal somites, some fossil forms and the larva nine. Marine, in very shallow waters, crawl, burrow in sand, can swim with gills. Feed on non-shelled molluscs and worms. Neolimulus.—Nine abdominal segments, Silurian. Hemiaspis.—Eight abdominal segments, Silurian. Belinurus.—Eight abdominal segments, last two or three fused. Carboniferous. Euproöps (=Prestwichia).—Seven abdominal segments. Carboni­ ferous. Limulus.—Six abdominal segments. Triassic to Recent. PANTOPODA (Nobody Crabs) Almost radial in appearance, with very small trunk and very long legs into which the gut (mesenteron) extends ; p r o s o m a only (cf. arach­ nids), the o p i s t h o s o m a represented only by genital operculum seg­ ment, which bears the last pair of legs, four pairs in all. Limbs not forked. No antennae. Mouth with chelicerae and pedipalps as in arachnids. No respiratory organs other t h a n the skin. Four simple eyes. Exclusively marine, shallow to deep seas, largest in deeper waters, reach two feet from toe to toe. Crawl among hydroids and seaweed. Adult cannot swim. Larval form in some has first three pairs of appendages=first three of adult, greater limbs added later. Pycnogonum.—Sucks sea anemones, British. TRILOBITA (Trilobites) Only known as fossils. So called from the three longitudinal lobes into which the body was divided, the middle = axial lobe, the l a t e r a l = pleural, acting as a cover for the limbs, which long remained unknown. Axial part consisted of head, thorax, and abdomen, with little differ­ entiation, as apart from the antennae all the limbs were biramous and used both for progression and as jaws. In the abdominal region ( = p y g i d i u m ) the segments were fused. There was no distinct telson, the pointed tail end being called the mucro. Head U-shaped, its median part (glabella) few-segmented, its lateral p a r t divided by a groove separating free cheeks in front from fixed cheeks behind. Compound eyes, sometimes reduced. Cambrian to Permian (only one genus in Permian and one family in Carboniferous). All marine, creeping, possibly swimming. HYPOPARIA.—Head suture marginal, so free cheeks and eyes not visible on upper surface (a larval condition in others). Harpes.

PISCES OPISTHOPARIA.—Free cheeks extend back to angles of eyes visible on upper surface. Olenellus, Paradoxides Olenus. (These three genera give their names to the divisions of the Cambrian : Lower, Middle, and Upper.) PROPARIA.—Free cheeks do not extend back to angles of eyes visible on upper surface. Staurocephalus.—Swelling in middle of head ; appeared in vician, when group attained its maximum.

CHAPTER

811 head, and three head, Ordo-

CXXXVIII

PISCES (Fishes) *

The chordates, animals with : (i) A notochord ; (2) Dorsal tubular nerve cord ; and (3) Gill-slits, are the second largest phylum, in number of species, in the animal kingdom. We have already dealt with the prochordates, constituting several sub-phyla, and there remains the C R A N I A T A = a n i m a l s with a cranium, practically synonymous with vertebrates, as all animals which have a cranium also have vertebrae, with the exception of the hag-fishes. The craniates or vertebrates were divided b y Haeckel into two sections : (1) Lower section, the lampreys and hag-fishes, without jaws ; (2) The higher section, including fishes, amphibians, reptiles, birds and mammals. To the first of these groups, called cyclostomes or marsipobranchs, the fossils known as ostracoderms were added, and the group was thereafter called :— AGNATHA (Jawless Vertebrates) The jaws are possibly represented by the sub-ocular arch below the eye and the tongue support, of cartilage in living forms. There are no paired appendages, or primitive pectoral lobes only. The living forms are eel-like, bare skinned, with no conus arteriosus to the heart, no pancreas or spleen. The fossils were armoured with bony plates and possibly had a bony endoskeleton. Medium size. PETROMYZONTIA.—Lampreys. Freshwater or coastal eel-like marine forms, ascending rivers to spawn, building nests with stones carried in mouth. They attach to fishes and rasp flesh (but do not invade body of victim), skin slimy, no scales (beneath head a reduced plate reminiscent of armour of ostracoderms), horny teeth in mouth and on tongue, mouth a ventral anterior funnel, two dorsal and one caudal fin, tail diphycercal. Nasohypophysial duct opening, on top of head, leads into nasal cavity, then on to tube representing anterior pituitary, no internal opening ; seven pairs of gill-pouches opening separately outside, kidney of adult = a mesonephros, brain with cerebrum * The plural of fish is fish when only one species is involved, fishes when more than one species.