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AMPHIOXUS
AMPHIOXUS
277
through anterior and posterior main branches to rest of body, the posterior supplying the visceral mass. E X C R E T O R Y SYSTEM.—Kidney single, almost solid, but with narrow space inside communicating— i. With pericardial cavity. 2. With ureter, a curved tube opening above and to right of pulmonary aperture. GLANDULAR SYSTEM.—Apparently all glands are associated with other systems. GENITAL SYSTEM.—Very complex. Ovotestis or h e r m a p h r o dite gland yellow, embedded in liver, right lobe, in second whorl from base. Eggs and sperms travel down sinuous white hermaphrodite duct, close to columella, which opens into cream-coloured a l b u m e n gland in upper half of basal whorl, between stomach and liver, left lobe. The c o m m o n duct leads off this. I t is partly divided inside into passages for eggs and sperms, which finally separate as distinct tubes, oviduct and spermiduct. Former leads into vagina, latter into protrusible penis, both of which have a common genital aperture. Opening into vagina also are— : i. Two tufts of tubular m u c o u s glands. 2. D a r t - s a c wherein a calcareous dart is formed. 3. Spermatheca, long, with side outgrowth, for reception of foreign sperm. Opening into penis is long flagellum, lying dorsal to crop. I n this the sperms are agglutinated into a long pencil : the s p e r m a t o phore. In pairing, the penis is inserted into the vagina of both pairing snails. Prior to pairing the dart is shot out a t the partner. Eggs telolecithal, holoblastic, development much modified.
CHAPTER
XXXIV
AMPHIOXUS (Lancelet)
DEFINITION.—The genus Amphioxus ( = Branchiostoma) was first described and assigned to a genus of molluscs by Pallas in 1774. Costa in 1834 discovered its relation to the lampreys, the larval form of which it resembles. Long regarded as a simple fish, it was placed by Haeckel, after extensive anatomical work by J. Müller and others, as a class of its own in the chordates, this subsequently being raised to a subphylum (see p. 270). On the whole it represents a very primitive type of chordate, especially in its development, first worked out in detail b y Kowalewsky. MODE O F LIFE.—Adults in shallow water with sandy bottom. Occur in some localities in the English Channel, more abundantly in Mediterranean and warmer seas. Swims actively, also burrows in sand, head or tail downward, in latter case with mouth exposed. Feeds on micro-organisms, e.g., diatoms
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278
Amphioxus—Mode of Life, continued.
brought in by a current of water. Sometimes lies on one side. Larvae pelagic. E X T E R N A L FEATURES.—Elongate, pointed at ends, flattened from side to side (compressed), not quite symmetrical {Figs. 189, 190). A deep body such as this is more common in vertebrates than in invertebrates. Length of adult i j inches to
Fig. 189.—Amphioxus.
tentacles
Adult, entire ; the gonads and myotomes show through the transparent skin.
with gill bars Fig. 190.—Amphioxus.
diverticulum (Myotomes not shown over whole animal.)
2 inches, smooth, pale brownish-white, dark in earlier stages, more or less transparent. The V-shaped blocks of muscle tissue show clearly along sides. Narrow dorsal fin along midline of back, continuous with wider caudal fin around tail. Below the latter is continued forward by a short narrow ventral fin, but along the anterior two-thirds the body is triangular in section, with sharp lower angles formed by the edges of the two metapleural folds. The latter are outgrowths or folds down from the side of the body which send a shelf-like ingrowth across the midline, where they meet, so as to enclose a large space, the atrium. The latter opens a t its hind end by the atriopore in the midline, just in front of the ventral fin, on segment 36. The atriopore must not be confused with the anus, which is found on the left side a t the base of the caudal fin. No gill-slits are visible from the outside, as they open into the atrial cavity. Thus the metapleural folds are comparable with the opercula of fishes, but are even more extensive. The mouth, too, does not open directly to the outside, as a space, the pre-oral hood, grows out around it. The entrance to the latter is somewhat ventral and is surrounded by 20-40 buccal cirri == tentacles with stiff supports. The true mouth is surrounded by a circular diaphragm, the velum, from which 12 velar tentacles arise.
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DERMAL SYSTEM.—The skin consists of :— 1. E p i d e r m i s of a single layer of short columnar epithelium, ciliated all over in the larva, retaining cilia over the buccal cirri in the adult. 2. D e r m i s , a very thin layer of connective tissue immediately below the preceding. The inside of the atrium is lined by skin as well as the outside of the pharynx. S K E L E T A L AND ARTICULAR SYSTEM.—No bones. I t consists of :— i. Notochord, a stiff elastic rod dorsal to the gut, traversing the body from end to end. 2. Buccal skeleton, curved rod of notochordal tissue in buccal hood with segmented supports for buccal tentacles. 3. F i n - r a y - b o x e s , small pockets of connective tissue, filled with cartilage, running in a single row just below the dorsal fin, and in a double row in the same relation to the ventral fin, much more numerous than the myotomes. 4. Gill-basket, of chitin-like substance, consisting of :— a. P r i m a r y - g i l l - b a r s in the primary arches, i.e., those originally present. They are united together above, forked below. b. S e c o n d a r y - g i l l - b a r s or t o n g u e - b a r s , in the secondary arches, which have been formed later than the primary, dividing them vertically. These are not forked below. c. C r o s s - b a r s or synapticulae lying in the cross-pieces, which run at right angles to and across the slits. Finally we must include— 5. A number of connective-tissue m e m b r a n e s , notably :— a. Thick notochordal sheath. b. Neural sheath around nerve tube. c. M y o c o m m a t a partitions between muscle segments, appearing < shaped at the surface. They alternate on the two sides of the animal. MUSCULAR SYSTEM.—Mostly of striated muscle. The bulk consists of :— 1. The lateral musculature, composed of 62 < -shaped segments or m y o t o m e s . They extend down into the metapleural folds. They are formed from the vertebral plate mesoderm and alternate on right and left sides, those on the right being a little in front of those on the left. 2. Below them the ventral musculature, not divided into segments, much thinner, in the floor of the atrium, formed from lateral plate mesoderm. 3. Splanchnic m u s c u l a t u r e in the gut wall, very slightly developed, distinguishable chiefly in the pharynx and around the mouth. NERVOUS SYSTEM.— 1. Tubular central nervous s y s t e m without true brain, the front end having a rather larger hollow space inside and termed cerebral vesicle and sometimes ' brain ' although it can only correspond with a fraction of the vertebrate brain. 2. Paired dorsal nerves.
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FORM AND STRUCTURE
Amphioxus—Nervous System, continued.
3. Paired ventral nerves. The two kinds of nerve, although apparently corresponding with dorsal and ventral nerveroots of vertebrates, do not join, and the dorsal root has no ganglion, its cells being widely scattered. Most of the fibres of the dorsal root are sensory, although it contains some motor fibres. The ventral root is purely motor, as in ventral roots of spinal nerves of vertebrates. The cord consists almost entirely of grey matter. The first two nerves are dorsal and have no corresponding ventral nerves. No sympathetic system occurs. SENSORY SYSTEM.—No paired organs. A large mass of pigment (eye-spot) at the front end of the nerve cord, and smaller spots scattered along its course may be connected with light perception. The skin, especially at the front end, and particularly in a pit on the left side thereof (Kolliker's pit), contains cells resembling those of olfactory epithelium. ALIMENTARY AND R E S P I R A T O R Y SYSTEMS.—Combined, water bringing in food and oxygen through the mouth (Fig. 191). The alimentary canal is a straight tube, from mouth to anus. Dorsal fin ray
Ventral root of spinal nerve
Dorsal root of spinal nerve
Epipharyngeal groove
Muscle in [metapleural fold
Fig. 191.—Amphioxus.
Transverse section of branchial region.
Its anterior portion, the pharynx, is a respiratory cavity. It is about equal in length to the narrower intestine which makes up the rest. Gill-slits open out from the walls of the pharynx, leading into the atrium. They are longitudinal clefts, divided during development into two vertical and four or five transverse orifices by tissue growing down and across. They run very obliquely, so a number are seen at the sides of the pharynx in a cross-section. Delicate ciliated epithelium lines them but there
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281
are no gills. From the front end of the intestine, on its underside, a blind outgrowth, the hepatic caecum, pushes forwards, carrying body wall before it, so as to lie in the atrium. Running longitudinally along the narrow floor of the pharynx is a groove, called the hypobranchial groove or endostyle, whilst a similar and parallel groove runs along the pharyngeal roof, the latter being known as the hyperbranchial or e p i branchial groove. In front of the gill-clefts, these grooves are united by lateral grooves around the velum. The cells lining these grooves are partly glandular and partly ciliated, the glandular secretion being mucus which lies in the groove, and which is propelled forwards in the endostyle by the action of the ciliated cells. From this lower groove the mucus passes forwards and upwards on either side t o the upper or epibranchial groove wherein it passes backwards. Many small food particles stick to this mucus and get carried back towards the intestine. CIRCULATORY SYSTEM.—Composed of small vessels which are not differentiated into arteries, veins and capillaries. Beneath the endostyle and parallel with it lies the ventral aorta, and from this lateral branchial v e s s e l s pass upward in the gill arches. J u s t above the level of the gill-clefts each branchial vessel breaks up into a network of small vessels called a g l o m u s , somewhat resembling the glomerulus of a kidney and, in fact, connected with an excretory organ. However, the branchial vessels of each side continue on to a branchial aorta, and the two branchial aortae run back along either side of the epipharyngeal groove to unite above the intestine to form a median trunk, the dorsal aorta. As in vertebrates generally, blood travels forwards in the ventral aorta and backwards in the dorsal aorta. There is no heart, however, but the lower ends of the branchial vessels are dilated and contractile and serve the same purpose. From the dorsal aorta blood is distributed over the intestine wall in branching vessels, which join a main trunk, the sub-intestinal vessel, running on the ventral side to the hepatic caecum. There branching occurs again, but collecting branches form the hepatic vessel, which carries the blood back to the ventral aorta. The sub-intestinal vessel is comparable with the portal vein and the hepatic vessel with the hepatic vein of typical vertebrates. The blood of A mphioxus is colourless, corpuscles of the leucocyte type alone being present. Lymph occurs in lymph spaces in parts not supplied by vessels, including the myotomes. E X C R E T O R Y SYSTEM.—Consists of paired tubules opening along the top of the atrium on each side. They correspond in number and position with the gill-slits and their vessels, and each is in close proximity to a glomus. Each tubule is a wide, bent tube, with outgrowths on its convex side. In the end of each of these outgrowths are a number of very narrow tubes each ending in a rounded solenocyte or flame cell. The latter is a cell with a long flagellum inside a tube. In fact, the whole of the narrow tube is an outgrowth of this cell. Such cells are found in worms (q.v.), and their presence in Amphioxus, established by Goodrich, shows t h a t in the excretory organs of this animal are
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FORM AND STRUCTURE
Amphioxus—Excretory System, continued.
protonephridia as in some worms, and not tubules of the type seen in typical vertebrates. Certain other structures are said to have a renal function in Amphioxus but they are of little importance. (See p . 255.) GLANDULAR SYSTEM.—Does not, apparently, exist as a separate entity in Amphioxus, but the endostyle is homologous with the thyroid gland of vertebrates (see LAMPREY), and a small pit,
Fig. 192.—Stages in development of Amphioxus. 1, 2, 3, Stages in gastrulation in longitudinal section ; 4, 5, Post-gastrula stage in transverse section ; 6, Post-gastrula stage in longitudinal section ; 7, Larva in longitudinal section.
lined by special cells below the brain, is a rudiment of the posterior pituitary. GENITAL SYSTEM.—Distinct male and female individuals exist, but they are not distinguishable externally. Paired g o n a d s (distinguished by microscopic structure as t e s t e s in the male, ovaries in the female) occur along the inner sides of the metapleural folds in segments 10 to 35, corresponding with and like the myotomes, alternating on the two sides. When ripe they burst through the body wall to shed their contents into the atrium, from which the contents escape to the outside, fertilization occurring in the open sea. Spawning occurs from April to July, in the evenings.
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283
DEVELOPMENT.—After fertilization a thin membrane is formed on the outside, within which the embryo develops for 8 to 15 hours, by which time not only e c t o d e r m and e n d o d e r m have formed, but also a neural plate and two m e s o d e r m a l s o m i t e s (Fig, 192). At this stage it bursts out to form a free swimming larva. The embryonic period is relatively short as compared with the enclosed stage in many other animals. The first cleavage separates two equal blastomeres, which subsequent events show represent the right and left halves of the body, the cleavage being in the median plane. The next two divisions are somewhat unequal and separate four larger cells of the vegetative pole from four smaller cells of the animal pole. After the 8-celled stage cleavage proceeds more rapidly in the animal hemisphere. The increase in number of cells causes a cavity to appear between them (the blastocœle or segmentation cavity). The organism is now a bias tula. The animal pole is marked by the persistent polar body. It is antero-ventral in position as regards the adult body, but is in the midline. By invagination of the cells of the vegetal hemisphere into those of the animal hemisphere, the embryo becomes a gastrula, e c t o d e r m and e n d o d e r m thus being formed. Note bilateral symmetry now appears, for : (1) There is deeper progressive invagination towards the dorsal side. (2) Greater growth on the dorsal margin of the blastopore. (3) A slight cleft in the dorsal midline on the margin of the blastopore. Whilst the blastopore is closing, the embryo is elongating in the antero-posterior direction, the dorsal surface is flattening and a pair of folds (neural folds), are forming from the ectoderm in the dorsal midline. A strip of ectoderm, the neural plate, sinks down in the midline, between these folds, and later forms a tube (the neural tube), beneath the rest of the surface epithelium, the folds joining together above it. The neural tube is the embryonic spinal cord, the main axis of the nervous system, which thus, in Amphioxus as in all vertebrates, is derived from ectoderm. It develops from in front backwards, and when the neural folds reach the blastopore they grow round it and come together below it, uniting above, so t h a t there is now a continuous canal (the neurenteric canal) leading from the neural tube into the archenteron. The anterior end of the neural tube remains open for a time by the neuropore, but this eventually closes. Whilst the aforementioned structures are being completed a strip of endoderm along the midline of the archenteron or future gut is being abstricted, a t first to form a tube, soon afterwards becoming a solid cord, the notochord, which thus lies parallel with and just below the neural tube. On both sides of the young notochord more tissue (mesoderm) is being budded off from the endoderm in the form of a groove which soon becomes distinct paired pouches. At the sides of the front of the gut gill-clefts are soon seen to be formed in pairs. They are formed by outgrowth of endoderm and
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FORM AND STRUCTURE
Amphioxus—Embryonic and Larval Development, continued.
ingrowth of ectoderm, in longitudinal grooves, the middle of which, in both layers, eventually becomes absorbed. At this stage all the main features of the adult Amphioxus are already outlined, and when it escapes with ciliated epithelium on the surface, to live an active animal life it does suggest, even better than the adult form, a type from which one may imagine vertebrates to be derived. The paired pouches formed by the mesoderm are called m y o coelomic pouches. They separate completely from the endoderm, and are found lying in pairs on each side. The first two pairs never form part of the original groove, but are distinct from their origin. The first pair, however, form a t the head end of the endodermal tube as a bilobed pouch. The first two pairs play no important part in the development of Amphioxus.* Each typical myoccelomic pouch divides into an upper and a lower cavity, each quite distinct. In the upper cavity the walls thicken and the inside space disappears. Its cells become converted to striped muscle fibres. The upper cavity is thus converted into a muscle-segment or m y o t o m e . The lower cavity expands, and its wall remains a thin plate of cells. Each lower cavity unites below the archenteron with its partner of the opposite side, by the breakdown of the cell layers between the two cavities. Each lower cavity also unites with the corresponding cavity in front and behind it. In this way a large space, the coelom, is formed below and at the sides of the archenteron. The layer of cells in contact with the endoderm constitutes the splanchnic m e s o d e r m ; the layer in contact with the ectoderm forms the s o m a t i c m e s o d e r m . The vessels arise by folding in these layers of mesoderm, skeletal tissue by differentiation therefrom. The ectoderm is at first ciliated, but loses this character in the adult stage. Pits of ectoderm ( s t o m o d œ u m and proctodaeum) contribute to the adult alimentary canal a t its front and hind ends, perforating to the archenteron and forming respectively mouth and anus.f The gill-slits arise in a rather similar way, but seem to be initiated by outgrowths of endoderm. Along the slit the coelom and mesoderm layers are obliterated. Ectoderm comes into contact with endoderm and then breakdown of the two occurs in the line of the slit. Each primary slit then becomes divided by a downgrowth, the tongue-bar, and then the cross pieces grow across. { The * The first pair correspond with the proboscis cavity of Balanoglossus, hence it is of interest to find they are united at first. The right one becomes the c a v i t y of t h e s n o u t , the left the p r e - o r a l pit ( H a t s c h e k ' s ) . The second pair form the c o l l a r c a v i t i e s which are also separate sections of coelom at the front end, also occurring in Balanoglossus. The upper part of each of these also forms a muscle mass (first m y o t o m e ) . t Both mouth and anus are asymmetrical in origin, being found on the left side. T h e mouth however shifts over to a median position. The oral hood is a later extension around the mouth. X Marked asymmetry characterizes the early appearance of the gill-slits. The first gill-slit is ventral, then goes to the right side. Additional gill-slits occur ventrally. They shift towards the right and eight others appear above them. Then some of the lower row disappear, eight remaining, and these shift to the left. The number then increases on each side.
ARTHROPODS
285
metapleural folds commence whilst there are only a few gill-slits. They are single downward folds of body-wall, on either side. They are covered with ectoderm. Each at first contains cœlomic mesoderm, from which a plate of ventral m u s c l e is differentiated. Later the lower ends of myotomes grow down into the upper parts of the metapleural folds. From the lower margin of each metapleural fold a shelf-like ingrowth is formed. By the meeting of the two outgrowths from each side the atrium is completed. Therefore the a trial cavity is lined on the inside, as well as outside, by ectoderm. The gonads arise from somatic mesoderm on the inner side of each metapleural fold.
CHAPTER
XXXV
ARTHROPODS DEFINITION.—A single phylum. A R T H R O P O D A . the largest in the animal kingdom, includes marine, freshwater, land, and aerial forms. (Insects, the largest group, exceed in numbers of species all other animals.) Triploblastic, coelomate animals, in which the ccelom is in embryonic development replaced by a blood-cavity formed by the fusion of large veins (hœmocœle). Segmentation of body visible externally, limbs also jointed. Hard exoskeleton, inhibiting growth, which occurs only a t special moulting periods when this is thrown off (ecdysis). Periods between growth termed instars. Two types will be outlined, a crustacean, Astacus, and an insect, Periplaneta. ASTACUS (Crayfish) MODE O F LIFE.—Crawls and swims on bottom of calcareous streams, sometimes suddenly swimming backward by jerks of flat tail-like hind end. Seizes large food, mostly carrion, with pincers, breaks it with smaller appendages, final breaking up in gizzard. Burrows under banks by day, mostly coming out by night. E X T E R N A L FEATURES.—Greenish-grey body, reaching 8 in. long, usually less (Fig. 193). Body divided into head, thorax, and abdomen, two former somewhat united to form céphalothorax. Nineteen pairs of appendages, corresponding with segments, apart from last, telson, without appendages. On head, two pairs of feelers or sensory appendages, three pairs of jaws, which here are appendages. On thorax eight pairs, of which the first three are intermediate between jaws and legs. The next are the large pincers, the others walking legs. On the abdomen six pairs, of which the first five are swimming organs of a small type, the last pair large and flat. Each appendage in the embryonic state is forked, or Y-shaped, consisting of : (1) A
277
through anterior and posterior main branches to rest of body, the posterior supplying the visceral mass. E X C R E T O R Y SYSTEM.—Kidney single, almost solid, but with narrow space inside communicating— i. With pericardial cavity. 2. With ureter, a curved tube opening above and to right of pulmonary aperture. GLANDULAR SYSTEM.—Apparently all glands are associated with other systems. GENITAL SYSTEM.—Very complex. Ovotestis or h e r m a p h r o dite gland yellow, embedded in liver, right lobe, in second whorl from base. Eggs and sperms travel down sinuous white hermaphrodite duct, close to columella, which opens into cream-coloured a l b u m e n gland in upper half of basal whorl, between stomach and liver, left lobe. The c o m m o n duct leads off this. I t is partly divided inside into passages for eggs and sperms, which finally separate as distinct tubes, oviduct and spermiduct. Former leads into vagina, latter into protrusible penis, both of which have a common genital aperture. Opening into vagina also are— : i. Two tufts of tubular m u c o u s glands. 2. D a r t - s a c wherein a calcareous dart is formed. 3. Spermatheca, long, with side outgrowth, for reception of foreign sperm. Opening into penis is long flagellum, lying dorsal to crop. I n this the sperms are agglutinated into a long pencil : the s p e r m a t o phore. In pairing, the penis is inserted into the vagina of both pairing snails. Prior to pairing the dart is shot out a t the partner. Eggs telolecithal, holoblastic, development much modified.
CHAPTER
XXXIV
AMPHIOXUS (Lancelet)
DEFINITION.—The genus Amphioxus ( = Branchiostoma) was first described and assigned to a genus of molluscs by Pallas in 1774. Costa in 1834 discovered its relation to the lampreys, the larval form of which it resembles. Long regarded as a simple fish, it was placed by Haeckel, after extensive anatomical work by J. Müller and others, as a class of its own in the chordates, this subsequently being raised to a subphylum (see p. 270). On the whole it represents a very primitive type of chordate, especially in its development, first worked out in detail b y Kowalewsky. MODE O F LIFE.—Adults in shallow water with sandy bottom. Occur in some localities in the English Channel, more abundantly in Mediterranean and warmer seas. Swims actively, also burrows in sand, head or tail downward, in latter case with mouth exposed. Feeds on micro-organisms, e.g., diatoms
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278
Amphioxus—Mode of Life, continued.
brought in by a current of water. Sometimes lies on one side. Larvae pelagic. E X T E R N A L FEATURES.—Elongate, pointed at ends, flattened from side to side (compressed), not quite symmetrical {Figs. 189, 190). A deep body such as this is more common in vertebrates than in invertebrates. Length of adult i j inches to
Fig. 189.—Amphioxus.
tentacles
Adult, entire ; the gonads and myotomes show through the transparent skin.
with gill bars Fig. 190.—Amphioxus.
diverticulum (Myotomes not shown over whole animal.)
2 inches, smooth, pale brownish-white, dark in earlier stages, more or less transparent. The V-shaped blocks of muscle tissue show clearly along sides. Narrow dorsal fin along midline of back, continuous with wider caudal fin around tail. Below the latter is continued forward by a short narrow ventral fin, but along the anterior two-thirds the body is triangular in section, with sharp lower angles formed by the edges of the two metapleural folds. The latter are outgrowths or folds down from the side of the body which send a shelf-like ingrowth across the midline, where they meet, so as to enclose a large space, the atrium. The latter opens a t its hind end by the atriopore in the midline, just in front of the ventral fin, on segment 36. The atriopore must not be confused with the anus, which is found on the left side a t the base of the caudal fin. No gill-slits are visible from the outside, as they open into the atrial cavity. Thus the metapleural folds are comparable with the opercula of fishes, but are even more extensive. The mouth, too, does not open directly to the outside, as a space, the pre-oral hood, grows out around it. The entrance to the latter is somewhat ventral and is surrounded by 20-40 buccal cirri == tentacles with stiff supports. The true mouth is surrounded by a circular diaphragm, the velum, from which 12 velar tentacles arise.
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279
DERMAL SYSTEM.—The skin consists of :— 1. E p i d e r m i s of a single layer of short columnar epithelium, ciliated all over in the larva, retaining cilia over the buccal cirri in the adult. 2. D e r m i s , a very thin layer of connective tissue immediately below the preceding. The inside of the atrium is lined by skin as well as the outside of the pharynx. S K E L E T A L AND ARTICULAR SYSTEM.—No bones. I t consists of :— i. Notochord, a stiff elastic rod dorsal to the gut, traversing the body from end to end. 2. Buccal skeleton, curved rod of notochordal tissue in buccal hood with segmented supports for buccal tentacles. 3. F i n - r a y - b o x e s , small pockets of connective tissue, filled with cartilage, running in a single row just below the dorsal fin, and in a double row in the same relation to the ventral fin, much more numerous than the myotomes. 4. Gill-basket, of chitin-like substance, consisting of :— a. P r i m a r y - g i l l - b a r s in the primary arches, i.e., those originally present. They are united together above, forked below. b. S e c o n d a r y - g i l l - b a r s or t o n g u e - b a r s , in the secondary arches, which have been formed later than the primary, dividing them vertically. These are not forked below. c. C r o s s - b a r s or synapticulae lying in the cross-pieces, which run at right angles to and across the slits. Finally we must include— 5. A number of connective-tissue m e m b r a n e s , notably :— a. Thick notochordal sheath. b. Neural sheath around nerve tube. c. M y o c o m m a t a partitions between muscle segments, appearing < shaped at the surface. They alternate on the two sides of the animal. MUSCULAR SYSTEM.—Mostly of striated muscle. The bulk consists of :— 1. The lateral musculature, composed of 62 < -shaped segments or m y o t o m e s . They extend down into the metapleural folds. They are formed from the vertebral plate mesoderm and alternate on right and left sides, those on the right being a little in front of those on the left. 2. Below them the ventral musculature, not divided into segments, much thinner, in the floor of the atrium, formed from lateral plate mesoderm. 3. Splanchnic m u s c u l a t u r e in the gut wall, very slightly developed, distinguishable chiefly in the pharynx and around the mouth. NERVOUS SYSTEM.— 1. Tubular central nervous s y s t e m without true brain, the front end having a rather larger hollow space inside and termed cerebral vesicle and sometimes ' brain ' although it can only correspond with a fraction of the vertebrate brain. 2. Paired dorsal nerves.
280
FORM AND STRUCTURE
Amphioxus—Nervous System, continued.
3. Paired ventral nerves. The two kinds of nerve, although apparently corresponding with dorsal and ventral nerveroots of vertebrates, do not join, and the dorsal root has no ganglion, its cells being widely scattered. Most of the fibres of the dorsal root are sensory, although it contains some motor fibres. The ventral root is purely motor, as in ventral roots of spinal nerves of vertebrates. The cord consists almost entirely of grey matter. The first two nerves are dorsal and have no corresponding ventral nerves. No sympathetic system occurs. SENSORY SYSTEM.—No paired organs. A large mass of pigment (eye-spot) at the front end of the nerve cord, and smaller spots scattered along its course may be connected with light perception. The skin, especially at the front end, and particularly in a pit on the left side thereof (Kolliker's pit), contains cells resembling those of olfactory epithelium. ALIMENTARY AND R E S P I R A T O R Y SYSTEMS.—Combined, water bringing in food and oxygen through the mouth (Fig. 191). The alimentary canal is a straight tube, from mouth to anus. Dorsal fin ray
Ventral root of spinal nerve
Dorsal root of spinal nerve
Epipharyngeal groove
Muscle in [metapleural fold
Fig. 191.—Amphioxus.
Transverse section of branchial region.
Its anterior portion, the pharynx, is a respiratory cavity. It is about equal in length to the narrower intestine which makes up the rest. Gill-slits open out from the walls of the pharynx, leading into the atrium. They are longitudinal clefts, divided during development into two vertical and four or five transverse orifices by tissue growing down and across. They run very obliquely, so a number are seen at the sides of the pharynx in a cross-section. Delicate ciliated epithelium lines them but there
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281
are no gills. From the front end of the intestine, on its underside, a blind outgrowth, the hepatic caecum, pushes forwards, carrying body wall before it, so as to lie in the atrium. Running longitudinally along the narrow floor of the pharynx is a groove, called the hypobranchial groove or endostyle, whilst a similar and parallel groove runs along the pharyngeal roof, the latter being known as the hyperbranchial or e p i branchial groove. In front of the gill-clefts, these grooves are united by lateral grooves around the velum. The cells lining these grooves are partly glandular and partly ciliated, the glandular secretion being mucus which lies in the groove, and which is propelled forwards in the endostyle by the action of the ciliated cells. From this lower groove the mucus passes forwards and upwards on either side t o the upper or epibranchial groove wherein it passes backwards. Many small food particles stick to this mucus and get carried back towards the intestine. CIRCULATORY SYSTEM.—Composed of small vessels which are not differentiated into arteries, veins and capillaries. Beneath the endostyle and parallel with it lies the ventral aorta, and from this lateral branchial v e s s e l s pass upward in the gill arches. J u s t above the level of the gill-clefts each branchial vessel breaks up into a network of small vessels called a g l o m u s , somewhat resembling the glomerulus of a kidney and, in fact, connected with an excretory organ. However, the branchial vessels of each side continue on to a branchial aorta, and the two branchial aortae run back along either side of the epipharyngeal groove to unite above the intestine to form a median trunk, the dorsal aorta. As in vertebrates generally, blood travels forwards in the ventral aorta and backwards in the dorsal aorta. There is no heart, however, but the lower ends of the branchial vessels are dilated and contractile and serve the same purpose. From the dorsal aorta blood is distributed over the intestine wall in branching vessels, which join a main trunk, the sub-intestinal vessel, running on the ventral side to the hepatic caecum. There branching occurs again, but collecting branches form the hepatic vessel, which carries the blood back to the ventral aorta. The sub-intestinal vessel is comparable with the portal vein and the hepatic vessel with the hepatic vein of typical vertebrates. The blood of A mphioxus is colourless, corpuscles of the leucocyte type alone being present. Lymph occurs in lymph spaces in parts not supplied by vessels, including the myotomes. E X C R E T O R Y SYSTEM.—Consists of paired tubules opening along the top of the atrium on each side. They correspond in number and position with the gill-slits and their vessels, and each is in close proximity to a glomus. Each tubule is a wide, bent tube, with outgrowths on its convex side. In the end of each of these outgrowths are a number of very narrow tubes each ending in a rounded solenocyte or flame cell. The latter is a cell with a long flagellum inside a tube. In fact, the whole of the narrow tube is an outgrowth of this cell. Such cells are found in worms (q.v.), and their presence in Amphioxus, established by Goodrich, shows t h a t in the excretory organs of this animal are
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Amphioxus—Excretory System, continued.
protonephridia as in some worms, and not tubules of the type seen in typical vertebrates. Certain other structures are said to have a renal function in Amphioxus but they are of little importance. (See p . 255.) GLANDULAR SYSTEM.—Does not, apparently, exist as a separate entity in Amphioxus, but the endostyle is homologous with the thyroid gland of vertebrates (see LAMPREY), and a small pit,
Fig. 192.—Stages in development of Amphioxus. 1, 2, 3, Stages in gastrulation in longitudinal section ; 4, 5, Post-gastrula stage in transverse section ; 6, Post-gastrula stage in longitudinal section ; 7, Larva in longitudinal section.
lined by special cells below the brain, is a rudiment of the posterior pituitary. GENITAL SYSTEM.—Distinct male and female individuals exist, but they are not distinguishable externally. Paired g o n a d s (distinguished by microscopic structure as t e s t e s in the male, ovaries in the female) occur along the inner sides of the metapleural folds in segments 10 to 35, corresponding with and like the myotomes, alternating on the two sides. When ripe they burst through the body wall to shed their contents into the atrium, from which the contents escape to the outside, fertilization occurring in the open sea. Spawning occurs from April to July, in the evenings.
AMPHIOXUS
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DEVELOPMENT.—After fertilization a thin membrane is formed on the outside, within which the embryo develops for 8 to 15 hours, by which time not only e c t o d e r m and e n d o d e r m have formed, but also a neural plate and two m e s o d e r m a l s o m i t e s (Fig, 192). At this stage it bursts out to form a free swimming larva. The embryonic period is relatively short as compared with the enclosed stage in many other animals. The first cleavage separates two equal blastomeres, which subsequent events show represent the right and left halves of the body, the cleavage being in the median plane. The next two divisions are somewhat unequal and separate four larger cells of the vegetative pole from four smaller cells of the animal pole. After the 8-celled stage cleavage proceeds more rapidly in the animal hemisphere. The increase in number of cells causes a cavity to appear between them (the blastocœle or segmentation cavity). The organism is now a bias tula. The animal pole is marked by the persistent polar body. It is antero-ventral in position as regards the adult body, but is in the midline. By invagination of the cells of the vegetal hemisphere into those of the animal hemisphere, the embryo becomes a gastrula, e c t o d e r m and e n d o d e r m thus being formed. Note bilateral symmetry now appears, for : (1) There is deeper progressive invagination towards the dorsal side. (2) Greater growth on the dorsal margin of the blastopore. (3) A slight cleft in the dorsal midline on the margin of the blastopore. Whilst the blastopore is closing, the embryo is elongating in the antero-posterior direction, the dorsal surface is flattening and a pair of folds (neural folds), are forming from the ectoderm in the dorsal midline. A strip of ectoderm, the neural plate, sinks down in the midline, between these folds, and later forms a tube (the neural tube), beneath the rest of the surface epithelium, the folds joining together above it. The neural tube is the embryonic spinal cord, the main axis of the nervous system, which thus, in Amphioxus as in all vertebrates, is derived from ectoderm. It develops from in front backwards, and when the neural folds reach the blastopore they grow round it and come together below it, uniting above, so t h a t there is now a continuous canal (the neurenteric canal) leading from the neural tube into the archenteron. The anterior end of the neural tube remains open for a time by the neuropore, but this eventually closes. Whilst the aforementioned structures are being completed a strip of endoderm along the midline of the archenteron or future gut is being abstricted, a t first to form a tube, soon afterwards becoming a solid cord, the notochord, which thus lies parallel with and just below the neural tube. On both sides of the young notochord more tissue (mesoderm) is being budded off from the endoderm in the form of a groove which soon becomes distinct paired pouches. At the sides of the front of the gut gill-clefts are soon seen to be formed in pairs. They are formed by outgrowth of endoderm and
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FORM AND STRUCTURE
Amphioxus—Embryonic and Larval Development, continued.
ingrowth of ectoderm, in longitudinal grooves, the middle of which, in both layers, eventually becomes absorbed. At this stage all the main features of the adult Amphioxus are already outlined, and when it escapes with ciliated epithelium on the surface, to live an active animal life it does suggest, even better than the adult form, a type from which one may imagine vertebrates to be derived. The paired pouches formed by the mesoderm are called m y o coelomic pouches. They separate completely from the endoderm, and are found lying in pairs on each side. The first two pairs never form part of the original groove, but are distinct from their origin. The first pair, however, form a t the head end of the endodermal tube as a bilobed pouch. The first two pairs play no important part in the development of Amphioxus.* Each typical myoccelomic pouch divides into an upper and a lower cavity, each quite distinct. In the upper cavity the walls thicken and the inside space disappears. Its cells become converted to striped muscle fibres. The upper cavity is thus converted into a muscle-segment or m y o t o m e . The lower cavity expands, and its wall remains a thin plate of cells. Each lower cavity unites below the archenteron with its partner of the opposite side, by the breakdown of the cell layers between the two cavities. Each lower cavity also unites with the corresponding cavity in front and behind it. In this way a large space, the coelom, is formed below and at the sides of the archenteron. The layer of cells in contact with the endoderm constitutes the splanchnic m e s o d e r m ; the layer in contact with the ectoderm forms the s o m a t i c m e s o d e r m . The vessels arise by folding in these layers of mesoderm, skeletal tissue by differentiation therefrom. The ectoderm is at first ciliated, but loses this character in the adult stage. Pits of ectoderm ( s t o m o d œ u m and proctodaeum) contribute to the adult alimentary canal a t its front and hind ends, perforating to the archenteron and forming respectively mouth and anus.f The gill-slits arise in a rather similar way, but seem to be initiated by outgrowths of endoderm. Along the slit the coelom and mesoderm layers are obliterated. Ectoderm comes into contact with endoderm and then breakdown of the two occurs in the line of the slit. Each primary slit then becomes divided by a downgrowth, the tongue-bar, and then the cross pieces grow across. { The * The first pair correspond with the proboscis cavity of Balanoglossus, hence it is of interest to find they are united at first. The right one becomes the c a v i t y of t h e s n o u t , the left the p r e - o r a l pit ( H a t s c h e k ' s ) . The second pair form the c o l l a r c a v i t i e s which are also separate sections of coelom at the front end, also occurring in Balanoglossus. The upper part of each of these also forms a muscle mass (first m y o t o m e ) . t Both mouth and anus are asymmetrical in origin, being found on the left side. T h e mouth however shifts over to a median position. The oral hood is a later extension around the mouth. X Marked asymmetry characterizes the early appearance of the gill-slits. The first gill-slit is ventral, then goes to the right side. Additional gill-slits occur ventrally. They shift towards the right and eight others appear above them. Then some of the lower row disappear, eight remaining, and these shift to the left. The number then increases on each side.
ARTHROPODS
285
metapleural folds commence whilst there are only a few gill-slits. They are single downward folds of body-wall, on either side. They are covered with ectoderm. Each at first contains cœlomic mesoderm, from which a plate of ventral m u s c l e is differentiated. Later the lower ends of myotomes grow down into the upper parts of the metapleural folds. From the lower margin of each metapleural fold a shelf-like ingrowth is formed. By the meeting of the two outgrowths from each side the atrium is completed. Therefore the a trial cavity is lined on the inside, as well as outside, by ectoderm. The gonads arise from somatic mesoderm on the inner side of each metapleural fold.
CHAPTER
XXXV
ARTHROPODS DEFINITION.—A single phylum. A R T H R O P O D A . the largest in the animal kingdom, includes marine, freshwater, land, and aerial forms. (Insects, the largest group, exceed in numbers of species all other animals.) Triploblastic, coelomate animals, in which the ccelom is in embryonic development replaced by a blood-cavity formed by the fusion of large veins (hœmocœle). Segmentation of body visible externally, limbs also jointed. Hard exoskeleton, inhibiting growth, which occurs only a t special moulting periods when this is thrown off (ecdysis). Periods between growth termed instars. Two types will be outlined, a crustacean, Astacus, and an insect, Periplaneta. ASTACUS (Crayfish) MODE O F LIFE.—Crawls and swims on bottom of calcareous streams, sometimes suddenly swimming backward by jerks of flat tail-like hind end. Seizes large food, mostly carrion, with pincers, breaks it with smaller appendages, final breaking up in gizzard. Burrows under banks by day, mostly coming out by night. E X T E R N A L FEATURES.—Greenish-grey body, reaching 8 in. long, usually less (Fig. 193). Body divided into head, thorax, and abdomen, two former somewhat united to form céphalothorax. Nineteen pairs of appendages, corresponding with segments, apart from last, telson, without appendages. On head, two pairs of feelers or sensory appendages, three pairs of jaws, which here are appendages. On thorax eight pairs, of which the first three are intermediate between jaws and legs. The next are the large pincers, the others walking legs. On the abdomen six pairs, of which the first five are swimming organs of a small type, the last pair large and flat. Each appendage in the embryonic state is forked, or Y-shaped, consisting of : (1) A