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Mechanoreceptors on the dorsal carapace of Limulus
Brain Research, 109 (1976) 615--622
615
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
Mechanoreceptors on the dorsal carapace of
Limulus
EHUD KAPLAN*, ROBERT B. BARLOW, JR., STEVEN C. CHAMBERLAIN AND DENNIS J. STELZNER Institute for Sensory Research, Syracuse University, Syracuse, N.Y. 13210 and (D.J.S.) Department of Anatomy, S.U.N.Y.- Upstate Medical Center, Syracuse, N. }I. (U.S.A.)
(Accepted March 5th, 1976)
During the course of our investigation of the lateral eye of Limulus in situ 12, we found that each eye is surrounded by hundreds of spine-like organs that discharge impulses in response to mechanical stimulation. These mechanoreceptors appear similar to but are larger than the common hair and spine receptors found in other arthropods. In addition to being located near the lateral eyes, the mechanoreceptors are distributed over the entire dorsal surface of the carapace with dense concentrations around the median eyes, near the base of sharp spines, on the margin, and on the telson. They appear to provide a major sensory input to the brain. Previous reports have been made of mechanoreceptors in Limulus. Wyse ~0 described mechanoreceptors in the claws and Eagles 8 investigated those in the lateral spines. Also, several studies have focused on the proprioceptors in the walking legs 1-a,16,2°. However, the mechanoreceptors that we found on the dorsal surface of the carapace appear to be in a separate class from those cited above. The purpose of this report is to describe, in brief, properties of the mechanoreceptor and to call attention to its potential use as a preparation in the study of mechanotransduction. To the unaided eye the mechanoreceptors appear as small pigmented pits on the dorsal surface of the carapace. Fig. 1 is a photograph of a section of shell bordering the lateral eye of an adult animal. In this area the mechanoreceptors are abundant and range in size from 60 to 180/~m. A similar range of sizes is found for the mechanoreceptors located around the median eyes, on the edges of the tail, along the margin of the shell, and near sharp ridges. Larger sizes (up to 300/~m in diameter) are found in other areas of the carapace. In Fig. 2 a scanning electron micrograph of a small section of carapace shows that the mechanoreceptor is situated in a pit. At the base of the pit a spine protrudes from a dome-like structure. The spine and the dome are the only parts of the mechanoreceptor exposed to the external environment, Both structures are less rigid than the surrounding shell and can be easily bent or deformed with a small probe. * Present address: The Rockefeller University, New York, N.Y., U.S.A.
616
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....
Fig. I. A p h o t o g r a p h o f m e c h a n o r e c e p t o r s l o c a t e d n e a r the d o r s a l e d g e o f a Limulus l a t e r a l eye.
The mechanoreceptors appear in the upper half of the photograph as small pigmented pits 60-180 /~m in diameter. Photoreceptor units (ommatidia) of the lateral eye are visible below the dark horizontal band where the edge of cornea joins the carapace. The ophthalmic ridge is located just beyond the field of view at the top of the photograph.
Fig. 3 shows t h a t a canal, 50-60 # m in d i a m e t e r , leads f r o m the base o f the pit t h r o u g h the chitin to the u n d e r l y i n g tissue (see legend for details o f the histological techniques). I n the u p p e r p a r t of the canal is a hair, 3-4 # m in d i a m e t e r , which has a hollow core. U n d e r light m i c r o s c o p y the core of the hair does not a p p e a r to be open at the tip o f the spine; however definite evidence on this p o i n t can only be o b t a i n e d
617
Fig. 2. An oblique view of the mechanoreceptor taken with a scanning electron microscope ( × 300). Slight wrinkles in the dome structure at the base of the spine are artifacts caused by the preparation of the tissue. by electron microscopy. Within the canal the hair is embedded in a chitinous substance which is continuous with the surrounding shell. At the base of the hair is a ring of 8-10 dendrites averaging 1-3 # m in diameter. The dendrites descend through the canal and their cell bodies form a small ganglion in a shallow depression on the inner surface of the shell. The cell bodies are oval, measuring 25-30/~m along their long
618
Fig. 3A. Transverse section through the carapace stained with methylene blue showing the various parts of the mechanoreceptor. The specimen was fixed in 1~ glutaraldehyde, postfixed in OsO4, and fiat-embeddedin Spurr's medium. Serial sections(1-2 pm) cut in transverse and longitudinal planes provided data for the line drawing in B. axes. Axons, 1-3 #m in diameter, leave the ganglion and run in a small bundle parallel to the surface of the shell. Bundles from neighboring receptors join together, forming large nerve trunks that enter the brain directly or through the ventral cord. Several other cell types in the micrograph in Fig. 3A are not indicated in the line drawing in Fig. 3B. Fig. 4 shows recordings of impulses discharged from an axon of a single mechanoreceptor (see legend for details). When the spine of the mechanoreceptor was in the upright position (upper trace), only infrequent impulses were recorded. Sudden displacements of the spine (middle two traces) elicited bursts of activity which died out within one or two seconds. These responses were not noticeably affected by the direction of displacement. Repetitive (sinusoidal) displacements of the spine elicited bursts of activity as indicated in the bottom trace. The receptor failed to follow stimuli at frequencies higher than approximately 6 Hz. Note that all the bursts contain about the same number of impulses, which further suggests that the mechanoreceptor is not directionally sensitive.
619
Fig. 3B. Line drawing of the mechanoreceptor summarizing the results of light microscopy. The 3 insets are magnified ( × 3) drawings of cross-sections of the hair and dendrites at different levels in the canal. The shading of the receptor and surrounding shell parts corresponds to the differential staining of the structure by methylene blue in A. Several cell types of trabeculae in the canal which appear in the photomicrograph in A are not shown in the line drawing.
The mechanoreceptor described in the report appears similar to the cuticular hairs found in all arthropods 5. However, the Limulus mechanoreceptor is larger than the c o m m o n hair receptors. It is also accessible and can be stimulated with precision. In addition, the mechanoreceptors are separated from one another by up to 1.5 m m and thus can be studied in relative isolation. What is the adequate stimulus for the mechanoreceptor? Responses near the inflection points of the sinusoidal stimuli in Fig. 4 suggest that the stimulus is the rate of displacement (velocity) of the spine. We note that similar results could have been produced by a rapidly adapting, displacement detector. However, there are other p o s s i b i l i t i e s - - a mechanoreceptor need not be either a velocity detector or a displacement detector. For example, the cockroach mechanoreceptor performs a 'fractional order differentation' of the stimulus n. We have not yet tested this possibility for the Limulus mechanoreceptor. The phasic responses shown in Fig. 4 are similar
620
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-.---4
L
I
I
I SEC Fig. 4. Spike activity recorded from a single mechanoreceptor. On the left are sketches of the mechanoreceptor in cross-section indicating the various modes of stimulation. On the right are 4 pairs of records from the oscilloscope. In each pair the lower record is the waveform of the voltage to the mechanical stimulator and the upper record is the response of a single nerve fiber. The recording method is similar to that used for recording from optic nerve fibers of the Liraulus lateral eye4. A hole, 3 cm in diameter, was cut in the shell about 2-3 cm anterior to one of the lateral eyes. The tissue surrounding one branch of a mechanoreceptor nerve was removed and the branch was dissected until a single active fiber was isolated. Recordings were made via a suction electrode with the animal partially submerged in ASW (Instant Ocean). No anesthetics were used. The stimulus was delivered by a heat-polished, glass pipette attached to a piezoelectric crystal (Bimorph, Clevite-Brush) driven by a function generator (No. 203A, Hewlett Packard). The slight asymmetry in the responses m the middle traces was probably caused by imperfect alignment of the stimulator.
621 to those reported by Eagles s for the mechanoreceptors in the lateral spines. We note, however, that the mechanoreceptors studied by Eagles are located beneath the carapace and appear not to be spine receptors. When active fibers from many mechanoreceptors were drawn into the suction electrode, stimulation of a single mechanoreceptor elicited only a single train of nerve impulses. This result is puzzling in view of the fact that each mechanoreceptor sends 8-10 axons to the brain. Apparently only one of the axons conducts impulses in the adult animal. The function of the other axons remains a mystery. A similar situation exists in the visual units (ommatidia) of the lateral eye of Limulus: each ommatidium sends 10-15 axons to the brain, but only one carries impulses. The inactivity of such a large percentage of sensory axons does not appear to be the result of trauma is. The function of the mechanoreceptors in regulating Limulus behavior is not known. They respond readily to water currents: gently touching the surface of the water with a small probe about 10 cm from the animal will excite a single receptor. We note that such stimuli produce some subsurface streaming but negligible pressure variations. The mechanoreceptors may thus help direct the animal's swimming movements. The high density of the receptors on the sharp ridges and on the margin of the shell suggests that they could also convey information about solid objects in the animal's path. Patten 15 found somewhat similar organs on the legs of Limulus and believed them to be temperature receptors. Fahrenbach 9 observed the canals of the mechanoreceptors on the internal surface of the shell and thought them to be gland ducts. Our experiments cannot rule out such functions for the receptors described in this report. Others have described neural responses to tactile stimulation to the legs 1 and gill box 18 of Limulus; however, no receptors were identified. The horseshoe crab is sometimes called the 'living fossil' since it has remained essentially unchanged for at least 350 million years a4. It is possible that the primitive nature of this animal is reflected in the structure and function of its sensory organs. Unfortunately, the fossil remains of Limulus are not well enough preserved to demonstrate the existence of mechanoreceptors. However, pigmented pits of the type that we have described for the Limulus mechanoreceptor have been observed in fossils of an extinct arthropod, Eurypterus remipes tetragonophthalmusL This arthropod is closely related to Limulus; in fact, the lateral eyes of Eurypterus are homologous to those of l_imulus17,19. The 'primitive' lateral eye of Limulus has been an admirable preparation for studies in visual physiology1°,1a. Perhaps the mechanoreceptor described in this report will be useful in the study of mechanotransduction. We thank Jozef Zwislocki for helpful suggestions and Judith Strauss for histological assistance. This work was supported by the National Institutes of Health (Grant EY00667). During part of this work, E.K. was a fellow of the Grass Foundation. 1 BARBER,S. B., Chemoreceptionand proprioception in Limulus, J. exp. Zool., 131 (1956) 51-74. 2 BARBER,S. B., Structure and properties of Limulus articular proprioceptors, J. exp. Zool., 143 (1960) 283-321.
622 3 BARBER,S. B., AND HAYES, W. F., A tendon receptor organ in Limulus, Comp. Biochem. Physh~L, ll (1964) 193-198. 4 BARLOW,R. B., JR., AND KAPLAN, E., Limalus lateral eye: properties of receptor units in the unexcised eye, Science, 174 (1971) 1027. 5 BULLOCK,T. H., AND HORRIDGE,G. A., Structure and Function in the Nervous Systems of Invertebrates, Freeman, San Francisco, 1965. 6 CHAPMAN, K. M., AND SMITH, R. W., Linear transfer function underlying impulse frequency modulation in a cockroach mechanoreceptor, Nature (Lond.), 197 (1963) 699-700. 7 CRONIN, T., personal communication. 8 EAGLES,D. A., Lateral spine mechanoreceptors in Limulus polyphemus, Comp. Biochem. Physiol., 44 (1973) 557-575. 9 FAHRENBACH, W. H., The visual system of the horseshoe crab Limulus polyphemus, Int. Rev. CytoL, 41 (1975) 285-349. 10 HARTLINE,H. K., Visual receptors and retinal interaction, Les Prix Nobel en 1967, Nobel Foundation, 1968 (1969), pp. 242-259. 11 HARTLINE,H. K., AND RATLIFF, F., Inhibitory interaction in the retina of Limulus. In M. G. F. FUORTES(Ed.), Handbook of Sensory Physiology, Vol. VII/2, Springer, Berlin, 1972, pp. 381-447. 12 KAPLAN,E., AND BARLOW,R. B., JR., Properties of visual cells in the lateral eye of Limulus in situ extracellular recordings, J. gen. Physiol., 66 (1975) 303-326. 13 LAHUE, R., AND CORNING, W. C., Habituation in Limulus abdominal ganglia, Biol. Bull., 140 (1971) 427439. 14 MOORE, R. C. (Ed.), Treatise on Invertebrate Paleontology, Part P, Arthropoda If, Geological Society of America, Boulder, Colo., 1955. 15 PATTEN,W., On the morphology and physiology of the brain and sense organs of Limulus, Quart. J. micr. Sci., 35 (1893) 1-96. 16 PRINGLE, J. W. S., Proprioception in Limulus, J. exp. Biol., 33 (1956) 658-667. 17 STORMER,L., Phylogeny and taxonomy of fossil horseshoe crabs, J. PalentoL, 26 (1952) 630-639. 18 WATERMAN,T. H., AND WIERSMA,C. A. G., The functional relation between retinal cells and optic nerve in Limulus, J. exp. Zool., 126 (1954) 59-85. 19 WILLS, L. J., A supplement to Gerhard Holm's '~ber die Organisation des Eyrupterus Fischeri Eichw' with special reference to the organs of sight, respiration and reproduction, Arkh. ZooL, 18 (1965-67) 93-145. 20 WYSE, G. A., Receptor organization and function in Limulus chelae, Z. vergl. PhysioL, 73 (1971) 249-273.
615
© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands
Mechanoreceptors on the dorsal carapace of
Limulus
EHUD KAPLAN*, ROBERT B. BARLOW, JR., STEVEN C. CHAMBERLAIN AND DENNIS J. STELZNER Institute for Sensory Research, Syracuse University, Syracuse, N.Y. 13210 and (D.J.S.) Department of Anatomy, S.U.N.Y.- Upstate Medical Center, Syracuse, N. }I. (U.S.A.)
(Accepted March 5th, 1976)
During the course of our investigation of the lateral eye of Limulus in situ 12, we found that each eye is surrounded by hundreds of spine-like organs that discharge impulses in response to mechanical stimulation. These mechanoreceptors appear similar to but are larger than the common hair and spine receptors found in other arthropods. In addition to being located near the lateral eyes, the mechanoreceptors are distributed over the entire dorsal surface of the carapace with dense concentrations around the median eyes, near the base of sharp spines, on the margin, and on the telson. They appear to provide a major sensory input to the brain. Previous reports have been made of mechanoreceptors in Limulus. Wyse ~0 described mechanoreceptors in the claws and Eagles 8 investigated those in the lateral spines. Also, several studies have focused on the proprioceptors in the walking legs 1-a,16,2°. However, the mechanoreceptors that we found on the dorsal surface of the carapace appear to be in a separate class from those cited above. The purpose of this report is to describe, in brief, properties of the mechanoreceptor and to call attention to its potential use as a preparation in the study of mechanotransduction. To the unaided eye the mechanoreceptors appear as small pigmented pits on the dorsal surface of the carapace. Fig. 1 is a photograph of a section of shell bordering the lateral eye of an adult animal. In this area the mechanoreceptors are abundant and range in size from 60 to 180/~m. A similar range of sizes is found for the mechanoreceptors located around the median eyes, on the edges of the tail, along the margin of the shell, and near sharp ridges. Larger sizes (up to 300/~m in diameter) are found in other areas of the carapace. In Fig. 2 a scanning electron micrograph of a small section of carapace shows that the mechanoreceptor is situated in a pit. At the base of the pit a spine protrudes from a dome-like structure. The spine and the dome are the only parts of the mechanoreceptor exposed to the external environment, Both structures are less rigid than the surrounding shell and can be easily bent or deformed with a small probe. * Present address: The Rockefeller University, New York, N.Y., U.S.A.
616
!
~
i!~! i
i!
....
Fig. I. A p h o t o g r a p h o f m e c h a n o r e c e p t o r s l o c a t e d n e a r the d o r s a l e d g e o f a Limulus l a t e r a l eye.
The mechanoreceptors appear in the upper half of the photograph as small pigmented pits 60-180 /~m in diameter. Photoreceptor units (ommatidia) of the lateral eye are visible below the dark horizontal band where the edge of cornea joins the carapace. The ophthalmic ridge is located just beyond the field of view at the top of the photograph.
Fig. 3 shows t h a t a canal, 50-60 # m in d i a m e t e r , leads f r o m the base o f the pit t h r o u g h the chitin to the u n d e r l y i n g tissue (see legend for details o f the histological techniques). I n the u p p e r p a r t of the canal is a hair, 3-4 # m in d i a m e t e r , which has a hollow core. U n d e r light m i c r o s c o p y the core of the hair does not a p p e a r to be open at the tip o f the spine; however definite evidence on this p o i n t can only be o b t a i n e d
617
Fig. 2. An oblique view of the mechanoreceptor taken with a scanning electron microscope ( × 300). Slight wrinkles in the dome structure at the base of the spine are artifacts caused by the preparation of the tissue. by electron microscopy. Within the canal the hair is embedded in a chitinous substance which is continuous with the surrounding shell. At the base of the hair is a ring of 8-10 dendrites averaging 1-3 # m in diameter. The dendrites descend through the canal and their cell bodies form a small ganglion in a shallow depression on the inner surface of the shell. The cell bodies are oval, measuring 25-30/~m along their long
618
Fig. 3A. Transverse section through the carapace stained with methylene blue showing the various parts of the mechanoreceptor. The specimen was fixed in 1~ glutaraldehyde, postfixed in OsO4, and fiat-embeddedin Spurr's medium. Serial sections(1-2 pm) cut in transverse and longitudinal planes provided data for the line drawing in B. axes. Axons, 1-3 #m in diameter, leave the ganglion and run in a small bundle parallel to the surface of the shell. Bundles from neighboring receptors join together, forming large nerve trunks that enter the brain directly or through the ventral cord. Several other cell types in the micrograph in Fig. 3A are not indicated in the line drawing in Fig. 3B. Fig. 4 shows recordings of impulses discharged from an axon of a single mechanoreceptor (see legend for details). When the spine of the mechanoreceptor was in the upright position (upper trace), only infrequent impulses were recorded. Sudden displacements of the spine (middle two traces) elicited bursts of activity which died out within one or two seconds. These responses were not noticeably affected by the direction of displacement. Repetitive (sinusoidal) displacements of the spine elicited bursts of activity as indicated in the bottom trace. The receptor failed to follow stimuli at frequencies higher than approximately 6 Hz. Note that all the bursts contain about the same number of impulses, which further suggests that the mechanoreceptor is not directionally sensitive.
619
Fig. 3B. Line drawing of the mechanoreceptor summarizing the results of light microscopy. The 3 insets are magnified ( × 3) drawings of cross-sections of the hair and dendrites at different levels in the canal. The shading of the receptor and surrounding shell parts corresponds to the differential staining of the structure by methylene blue in A. Several cell types of trabeculae in the canal which appear in the photomicrograph in A are not shown in the line drawing.
The mechanoreceptor described in the report appears similar to the cuticular hairs found in all arthropods 5. However, the Limulus mechanoreceptor is larger than the c o m m o n hair receptors. It is also accessible and can be stimulated with precision. In addition, the mechanoreceptors are separated from one another by up to 1.5 m m and thus can be studied in relative isolation. What is the adequate stimulus for the mechanoreceptor? Responses near the inflection points of the sinusoidal stimuli in Fig. 4 suggest that the stimulus is the rate of displacement (velocity) of the spine. We note that similar results could have been produced by a rapidly adapting, displacement detector. However, there are other p o s s i b i l i t i e s - - a mechanoreceptor need not be either a velocity detector or a displacement detector. For example, the cockroach mechanoreceptor performs a 'fractional order differentation' of the stimulus n. We have not yet tested this possibility for the Limulus mechanoreceptor. The phasic responses shown in Fig. 4 are similar
620
,<
-.---4
L
I
I
I SEC Fig. 4. Spike activity recorded from a single mechanoreceptor. On the left are sketches of the mechanoreceptor in cross-section indicating the various modes of stimulation. On the right are 4 pairs of records from the oscilloscope. In each pair the lower record is the waveform of the voltage to the mechanical stimulator and the upper record is the response of a single nerve fiber. The recording method is similar to that used for recording from optic nerve fibers of the Liraulus lateral eye4. A hole, 3 cm in diameter, was cut in the shell about 2-3 cm anterior to one of the lateral eyes. The tissue surrounding one branch of a mechanoreceptor nerve was removed and the branch was dissected until a single active fiber was isolated. Recordings were made via a suction electrode with the animal partially submerged in ASW (Instant Ocean). No anesthetics were used. The stimulus was delivered by a heat-polished, glass pipette attached to a piezoelectric crystal (Bimorph, Clevite-Brush) driven by a function generator (No. 203A, Hewlett Packard). The slight asymmetry in the responses m the middle traces was probably caused by imperfect alignment of the stimulator.
621 to those reported by Eagles s for the mechanoreceptors in the lateral spines. We note, however, that the mechanoreceptors studied by Eagles are located beneath the carapace and appear not to be spine receptors. When active fibers from many mechanoreceptors were drawn into the suction electrode, stimulation of a single mechanoreceptor elicited only a single train of nerve impulses. This result is puzzling in view of the fact that each mechanoreceptor sends 8-10 axons to the brain. Apparently only one of the axons conducts impulses in the adult animal. The function of the other axons remains a mystery. A similar situation exists in the visual units (ommatidia) of the lateral eye of Limulus: each ommatidium sends 10-15 axons to the brain, but only one carries impulses. The inactivity of such a large percentage of sensory axons does not appear to be the result of trauma is. The function of the mechanoreceptors in regulating Limulus behavior is not known. They respond readily to water currents: gently touching the surface of the water with a small probe about 10 cm from the animal will excite a single receptor. We note that such stimuli produce some subsurface streaming but negligible pressure variations. The mechanoreceptors may thus help direct the animal's swimming movements. The high density of the receptors on the sharp ridges and on the margin of the shell suggests that they could also convey information about solid objects in the animal's path. Patten 15 found somewhat similar organs on the legs of Limulus and believed them to be temperature receptors. Fahrenbach 9 observed the canals of the mechanoreceptors on the internal surface of the shell and thought them to be gland ducts. Our experiments cannot rule out such functions for the receptors described in this report. Others have described neural responses to tactile stimulation to the legs 1 and gill box 18 of Limulus; however, no receptors were identified. The horseshoe crab is sometimes called the 'living fossil' since it has remained essentially unchanged for at least 350 million years a4. It is possible that the primitive nature of this animal is reflected in the structure and function of its sensory organs. Unfortunately, the fossil remains of Limulus are not well enough preserved to demonstrate the existence of mechanoreceptors. However, pigmented pits of the type that we have described for the Limulus mechanoreceptor have been observed in fossils of an extinct arthropod, Eurypterus remipes tetragonophthalmusL This arthropod is closely related to Limulus; in fact, the lateral eyes of Eurypterus are homologous to those of l_imulus17,19. The 'primitive' lateral eye of Limulus has been an admirable preparation for studies in visual physiology1°,1a. Perhaps the mechanoreceptor described in this report will be useful in the study of mechanotransduction. We thank Jozef Zwislocki for helpful suggestions and Judith Strauss for histological assistance. This work was supported by the National Institutes of Health (Grant EY00667). During part of this work, E.K. was a fellow of the Grass Foundation. 1 BARBER,S. B., Chemoreceptionand proprioception in Limulus, J. exp. Zool., 131 (1956) 51-74. 2 BARBER,S. B., Structure and properties of Limulus articular proprioceptors, J. exp. Zool., 143 (1960) 283-321.
622 3 BARBER,S. B., AND HAYES, W. F., A tendon receptor organ in Limulus, Comp. Biochem. Physh~L, ll (1964) 193-198. 4 BARLOW,R. B., JR., AND KAPLAN, E., Limalus lateral eye: properties of receptor units in the unexcised eye, Science, 174 (1971) 1027. 5 BULLOCK,T. H., AND HORRIDGE,G. A., Structure and Function in the Nervous Systems of Invertebrates, Freeman, San Francisco, 1965. 6 CHAPMAN, K. M., AND SMITH, R. W., Linear transfer function underlying impulse frequency modulation in a cockroach mechanoreceptor, Nature (Lond.), 197 (1963) 699-700. 7 CRONIN, T., personal communication. 8 EAGLES,D. A., Lateral spine mechanoreceptors in Limulus polyphemus, Comp. Biochem. Physiol., 44 (1973) 557-575. 9 FAHRENBACH, W. H., The visual system of the horseshoe crab Limulus polyphemus, Int. Rev. CytoL, 41 (1975) 285-349. 10 HARTLINE,H. K., Visual receptors and retinal interaction, Les Prix Nobel en 1967, Nobel Foundation, 1968 (1969), pp. 242-259. 11 HARTLINE,H. K., AND RATLIFF, F., Inhibitory interaction in the retina of Limulus. In M. G. F. FUORTES(Ed.), Handbook of Sensory Physiology, Vol. VII/2, Springer, Berlin, 1972, pp. 381-447. 12 KAPLAN,E., AND BARLOW,R. B., JR., Properties of visual cells in the lateral eye of Limulus in situ extracellular recordings, J. gen. Physiol., 66 (1975) 303-326. 13 LAHUE, R., AND CORNING, W. C., Habituation in Limulus abdominal ganglia, Biol. Bull., 140 (1971) 427439. 14 MOORE, R. C. (Ed.), Treatise on Invertebrate Paleontology, Part P, Arthropoda If, Geological Society of America, Boulder, Colo., 1955. 15 PATTEN,W., On the morphology and physiology of the brain and sense organs of Limulus, Quart. J. micr. Sci., 35 (1893) 1-96. 16 PRINGLE, J. W. S., Proprioception in Limulus, J. exp. Biol., 33 (1956) 658-667. 17 STORMER,L., Phylogeny and taxonomy of fossil horseshoe crabs, J. PalentoL, 26 (1952) 630-639. 18 WATERMAN,T. H., AND WIERSMA,C. A. G., The functional relation between retinal cells and optic nerve in Limulus, J. exp. Zool., 126 (1954) 59-85. 19 WILLS, L. J., A supplement to Gerhard Holm's '~ber die Organisation des Eyrupterus Fischeri Eichw' with special reference to the organs of sight, respiration and reproduction, Arkh. ZooL, 18 (1965-67) 93-145. 20 WYSE, G. A., Receptor organization and function in Limulus chelae, Z. vergl. PhysioL, 73 (1971) 249-273.