Text Book of Biology Part 10

Section 2. Comparing the general build, of a dog-fish with that of a rabbit, we notice the absence of a distinct neck, and the general conical form; the presence of a large tail, as considerable, at first, in diameter as the hind portion of the body, and of the first importance in progression, in which function the four paddle-shaped limbs, the lateral fins, simply co-operate with the median fin along the back for the purpose of steering; and, as a consequence of the size of the tail, we note also the ventral position of the apertures of the body. The a.n.u.s, and urinary and genital ducts unite in one common chamber, the cloaca. Behind the head, and in front of the fore fin (pectoral fin), are five gill slits (g.s.) leading from the pharynx to the exterior. Just behind the eye is a smaller and more dorsal opening of the same kind, the spiracle (sp.). On the under side of the head, in front of the mouth, is the nasal aperture (olf.), the opening of the nasal sac, which, unlike the corresponding organ of the air-frequenting vertebrata, has no internal narial opening. There is, however, a groove running from olf. to the corner of the mouth, and this, closing, in the vertebrate types that live in air and are exposed to incessant evaporation of their lubricating secretions, const.i.tutes the primitive nasal pa.s.sage. The limbs are undifferentiated into upper, lower, and digital portions, and are simply jointed, flattened expansions.

Section 3. The skin of the dog-fish is closely set with pointed tooth-like scales, the placoid scales, and these are continued over the lips into the mouth as teeth. Each scale consists of a base of true bone, with a little tubercle of a harder substance, dentine, capped by a still denser covering, the enamel. The enamel is derived from the outer layer of the embryonic dog-fish, the epiblast, which also gives rise to the epidermis; while the dentine and bony base arise in the underlying mesoblast, the dermis. A mammalian tooth has essentially the same structure: an outer coat of enamel, derived from epiblast, overlies a ma.s.s of dentine, resting on bone, but the dentine is excavated internally, to form a pulpcavity containing blood-vessels and nerves. Most land animals, however, have teeth only in their mouths, and have lost altogether the external teeth which const.i.tute the armour of the dog-fish. Besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata, in the membrane bones. How placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog. This bone lies on the roof of the frog's mouth, and bears a number of denticles, and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent. In the salamander, behind the teeth-bearing vomers comes a similar toothed parasphenoid bone.

The same bone occurs in a corresponding position in the frog, but without teeth. In some tailed amphibians the vomers and splenials are known to arise by the fusion of small denticles. These facts seem to point to stages in the fusion of placoid bases, and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones, a process entirely completed in the mammalian type.

Section 4. The alimentary ca.n.a.l of the dog-fish, is a simple tube thrown into a Z shape. The mouth is rough with denticles, and has a fleshy immovable tongue on its floor. In the position of the Eustachian tube there is a pa.s.sage, the spiracle (sp.), running out to the exterior just external to the cartilage containing the ear. The pharynx communicates with the exterior through five gill slits (g.s.), and has, of course, no glottis or other lung opening. There is a wide oesophagus pa.s.sing into a U-shaped stomach (st.), having, like the rabbit's, the spleen (sp.) on its outer curvature. There is no coiling small intestine, but the short portion, receiving the bile duct (b.d.) and duct of the pancreas (pan.), is called the duodenum (d'dum.). The liver has large left (L.lv.) and right lobes, and a median lobe (M.lv.), in which the gall bladder (g.bl.) is embedded. The next segment of the intestine is fusiform, containing a spiral valve (Figure 4), the shelf of which points steeply forward; it is sometimes called the colon (co.). It is absorptive in function and probably represents morphologically, as it does physiologically, the greater portion of the small intestine. A rectal gland (r.g.) opens from the dorsal side into the final portion of the ca.n.a.l (r.e.c.t.u.m).

Section 5. The circulation presents, in many respects, an approximation to the state of affairs in the developing embryos of the higher types. The heart (Figure 3, Sheet -14- {Error in First Edition} [16]) is roughly, Z shape, and transmits only venous blood. It lies in a cavity, the pericardial cavity (P.c.c.), cut off by a part.i.tion from the general coelome. At one point this part.i.tion is imperfect, and the two s.p.a.ces communicate through a pericardio-peritoneal ca.n.a.l (p.p.c.), which is also indicated by an arrow (p.p.) in the position and direction in which the student, when dissecting, should thrust his "seeker," in Figure 1 Sheet 15. A sinus venosus (s.v. in Figure 3, Sheet 16) receives the venous trunks, and carries the blood through a valve into the baggy and transversely extended -auricle- [atrium] (au.), whence it pa.s.ses into the muscular ventricle (Vn.), and thence into the truncus arteriosus. This truncus consists of two parts: the first, the conus or pylangium (c.a.), muscular, contractile, and containing a series of valves; the second, the bulbus or synangium (b.a.), without valves and pulsatile. In the rabbit both sinus and truncus are absent, or merged in the adjacent parts of the heart.

Section 6. From the bulbus there branch, on either side, four arterial trunks, the first of which forks, so that altogether there are five afferent branchials (a.br.) taking blood to be aerated in the gills, here highly vascular filamentary outgrowths of the internal walls of the gill slits.

{Lines from Second Edition only.} [There are altogether nine vascular outgrowths (demi-branchs), one on each wall of each gill slit except the last, on the hind wall of which there is none. (In the spiracle is a miniature demibranch, the pseudo-branch. This suggests that the spiracle is really a somewhat modified gill slit.)]

Four efferent branchials (e.br.) carry the aerated blood on to the dorsal aorta (d.ao.). A carotid artery runs forward to the head, and a hypo-branchial artery supplies the ventral side of the pharyngeal region. There are sub-clavian, coeliac, mesenteric, and pelvic arteries, and the dorsal aorta is continued through the length of the tail as the caudal artery (Cd.A.).

Section 7. A caudal vein (Cd.V.), bringing blood back from the tail, splits behind the kidneys (K.), and forms the paired renal portal veins (r.p.v.), breaking up into a capillary system in the renal organ. A portal vein brings blood from the intestines to the liver.

Section 8. Instead of being tubular vessels, the chief veins of the dog-fish are, in many cases, irregular baggy sinuses. Three main venous trunks flow into the sinus venosus. In the median line from behind comes the hepatic sinus (H.S.); and laterally, from a dorsal direction, the Cuvierian sinuses (C.S.) enter it. These, as the student will presently perceive, are the equivalents of the rabbit's superior cavae. They receive, near their confluence with the sinus venosus, the inferior jugular vein (I.J.V.). At their dorsal origin, they are formed by the meeting of the anterior (A.C.S.) and posterior (P.C.S.) cardinal sinuses. The anterior cardinal sinus -is, roughly, the equivalent of the internal jugular vein-, lies along dorsal to the gill slits (g.s.), and receives an orbital sinus from the eye. The posterior cardinal sinus receives a sub-clavian vein (s.c.v.) and a lateral vein (L.V.), and fuses posteriorly with its fellow in the middle line. This median fusion is a departure from the normal fish type. It must not be confused with the inferior cava, which is not found in the dog-fish, the [right] posterior cardinals representing the rabbit's azygos vein. A simplified diagram of the circulation of a fish is given in Figure 2, Sheet 16, and this should be carefully compared with the corresponding small figure given of the vascular system of our other types.

{Lines from Second Edition only.} [The blood of the dog-fish resembles that of the frog.]

Section 9. The internal skeleton, as we have said, is entirely cartilaginous, and only those parts which are pre-formed in cartilage in the skeletons of the higher types are represented here. The spinal column consists of two types of vertebrae, the trunk, bearing short, distinct, horizontally-projecting ribs (r.), and the caudal. The diagrams of Figure 5 [(Sheet 18)] are to ill.u.s.trate the structure of the centrum of a dog-fish vertebra; C is a side view, D a horizontal median section, A and B are transverse sections at the points indicated by -B and A- [A and B] respectively in Figure C. -(By an unfortunate slip of the pen in the figure, A was subst.i.tuted for B; section A corresponds to line B, and vice versa.)- The vertebrae are hollowed out both anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord runs through the entire length of the vertebral column, being constricted at the centres of the centra, and dilated between them.

The neural arch above the centrum, and containing the spinal cord, is made up of neural plates (n.p.), and interneural plates (i.n.p.), completed above by a median neural spine (n.s.). In the caudal region, instead of ribs projecting outwardly, there are haemal processes, inclined downwards and meeting below, forming an arch, the haemal arch, containing the caudal artery and vein-- the vein ventral to the artery-- and resembling the neural arch, which contains the spinal cord above, in shape and size.

Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have only a very vague resemblance to the corresponding structures in the rabbit. The girdle (g.) is a transverse bar lying ventral to the pericardial wall, and sending up a portion (sc.), dorsal to the attachment of the limb, which answers to the scapula and supra-scapula of the forms above the fish. Three main cartilages, named respectively the pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the limb. With these, smaller cartilaginous plates, rods, and nodules articulate, and form a flattened skeletal support for the fin.

Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are similar in structure, but the pro-pterygium and meso-pterygium are absent, and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium. In the male, but not in the female, the pelvic fins are united behind the cloaca, and there are two stiff grooved copulatory organs, the claspers (cl. in Figure 1), which have a cartilaginous support (cl.c.). These claspers form the readiest means of determining the s.e.x of a specimen before dissection.

Section 12. The skull consists of a cartilaginous cranium, and of jaw and visceral arches. The cranium persists throughout life, in what closely resembles a transitory embryonic condition of the higher types. There is a nasal capsule (na.c.), a brain case proper, and lateral otic (auditory) capsules (ot.c.) containing the internal ear.

(This should be compared with the frog's embryonic skull.) The upper jaw has a great bar of cartilage, the palato-pterygoid, as its sole support; the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane bones-- is, of course, not represented. In the frog this bar of cartilage is joined directly to the otic capsule by a quadrate portion, but this is only doubtfully represented in the dog-fish by a nodule of cartilage in the pre-spiracular ligament (p.s.). The lower jaw is supported, by Meckel's cartilage (M.C.). The hyoid arch consists of two main ma.s.ses of cartilage, the hyomandibular (h.m.), and the ceratohyal (c.h.); the former of these is tilted slightly forward, so that the gill slit between it and the jaw arch is obliterated below, and the cartilage comes to serve as the intermediary in the suspension of the jaw from the otic ma.s.s. There are five branchia[l] arches, made up pharyngo-, epi- and cerato-branchials, and the ventral elements fuse in the middle line to form a common plate of cartilage. Outside these arches are certain small cartilages, the extra branchials (ex.b.) which, together with certain small l.a.b.i.als by the nostrils and at the sides of the gape, probably represent structures of considerably greater importance in that still more primitive fish, the lamprey. The deep groove figured lateral to the otic capsule is the connecting line of the orbital and anterior cardinal sinuses; the outline of the anterior cardinal sinus in this figure and in Figure 1 is roughly indicated by a dotted line.

Section 13. Figure 3a is a rough diagram of the internal ear-- the only auditory structure of our type (compare Rabbit, Sheet 7). To dissect out the auditory labyrinth without injury is a difficult performance, but its structure may be made out very satisfactorily by paring away successive slices of the otic ma.s.s. Such a section is shown by Figure 3b; through the translucent hyaline cartilage the utriculus and horizontal ca.n.a.l can be darkly seen. The ductus endolymphaticus (vide Rabbit) is indicated by a dotted line in our figure. It is situated internal to the right-angle between the two vertical ca.n.a.ls, and reaches to the surface of the otic capsule.

Section 14. The brain shows the three primary vesicles much more distinctly than do our higher types. The fore-brain has large laterally separated olfactory lobes (rh.), there are relatively small "hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and the gland itself is much nearer the surface, being embedded in the cartilage of the brain case, and the pituitary body is relatively very large, and has lateral vascular lobes on either side. Following the usual interpretation of the parts, we find optic lobes (op.l.) as the roof of the mid-brain, and behind a very large, median, hollow, tongue-shaped cerebellum (c.b.). The medulla is large, and certain lateral restiform tracts (r.t.) therein, which also occur in the higher types, are here exceptionally conspicuous.

Section 15. The dog-fish has ten pairs of cranial nerves, corresponding to the anterior ten of the rabbit very closely, when we allow for the modification the latter has suffered through the conversion of some part of the spiracular cleft to an eardrum, and the obliteration of the post-hyoid branchial slits.

The first and second nerves are really brain lobes, and nerves of the special senses of smell and sight respectively.

The third (oculomotor), the fourth (patheticus), and the sixth (abducens) are distributed to exactly the same muscles of the eyeball as they are in the rabbit.

The fifth nerve, has, in the dog-fish, as in the rabbit, three chief branches. V.2 and V.3 fork over the mouth just as they do in the mammal; V.1 pa.s.ses out of the cranium by a separate and more dorsal opening, and runs along a groove along the dorsal internal wall of the orbit, immediately beneath a similar branch of VII., which is not distinct in the rabbit. The grooves are shown in the figure of the cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is called the ophthalmic (oph.). It is distributed to the skin above the nose and orbit. When the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance, and on removing the tough skin over this region and over the centre of the skull he finds, lying beneath it, a quant.i.ty of coiling simple tubuli full of such yellowish matter.

These tubuli open on the surface by small pores, and the nerves terminate in hair-like extremities in their lining. These sense tubes are peculiar to aquatic forms; allied structures are found over the head and along a lateral line (see below) in the tadpole, but when the frog emerges from the water they are lost. They, doubtless, indicate some unknown sense entirely beyond our experience, and either only possible or only necessary when the animal is submerged.

In addition to the ophthalmic moiety mentioned above, the seventh nerve has a vidian branch (vid.) running over the roof of the mouth, and besides this its main branches fork over the spiracle, just as V. forks over the mouth, and as IX. and X. fork over gill clefts. This nerve in the rabbit is evidently considerably modified from this more primitive condition.

The eighth is the auditory nerve, as in the rabbit.

The ninth nerve forks over the first branchial cleft.

The tenth nerve is easily exposed by cutting down through the body wall muscles over the gill clefts, into the anterior cardinal sinus (A.C.S.). It gives off (a) branches forking over the posterior four gill slits, (b) a great lateral nerve running inward, and back through the body-wall muscle, and connected with a line of sense organs similar to those in the head, the lateral line, and (c) a visceral nerve curving round to the oesophagus and stomach. In dissection it becomes very evident that the tenth nerve is really a leash of nerves, each one equivalent to the ninth.

We may here call the attention of the reader to the fact of the singular resemblance of V., VII., IX., and the factors of X. That each has a ventral fork, we have already noticed. Each also (?IX.) has a dorsal const.i.tuent connected with the sense organs of the skin. The vidian branch of VII., however, is not evidently represented in the others.

Section 16. The coelom of the dog-fish is peculiar-- among the types we treat of-- in the possession of two direct communications with the exterior, in addition to the customary indirect way through the oviduct.

These are the abdominal pores (a.p.) on either side of the cloaca in either s.e.x. They can always be readily demonstrated by probing out from the body cavity, in the direction indicated by the arrow (a.p.) in Figure 1, Sheet 15. They probably serve to equalize the internal and external pressure of the fish as it changes its depth in the water, just as the Eustachian tubes equalize the pressure on either side of the mammal's tympanic membrane.

Section 17. The musculature of the dog-fish body is cut into V-shaped segments, the point of the V being directed forward. The segments alternate with the vertebrae, and are called myomeres.

Such a segmentation is evident, though less marked, in the body wall muscles of the frog, and in the abdominal musculature of the rabbit and other mammals it is still to be traced.

Section 18. The uro-genital organs of the female dog-fish (Figure 1, Sheet 17) consist of an unpaired ovary (ov.), paired oviducts (o.d.), enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.), with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the cloaca by a median urinary papilla separate from the oviducal openings. The eggs contain much yolk, and, like those of the fowl, are very large; like the fowl, too, one of the ovaries is suppressed, and it is the right ovary that alone remains. The two oviducts meet in front of the liver ventral to the oesophagus, and have there a common opening by which the ova are received after being shed into the body cavity.

The eggs receive an oblong h.o.r.n.y case in the oviduct; in the figure such a case is figured as distending the duct at e. The testes of the male (T. in Figure 2) are partially confluent in the middle line. They communicate through vasa efferentia (v.e.) with the modified anterior part of the kidney, the epididymis (ep.), from which the vas deferens (v.) runs to the median uro-genital sinus (u.g.s.), into which the ureters (ur.) also open. The silvery peritoneum (lining of the body cavity) covers over the reddish kidneys, and hides them in dissection.

Section 19. Figure 3, Sheet 17, is a generalized diagram of the uro-genital organs in the vertebrata; M.L. is the middle line of the body, G. is the genital organ, Pr. is the p.r.o.nephros, or fore kidney, a structure which is never developed in the dog-fish, but which has functional importance in the tadpole and cod, and appears as a transitory rudiment in the chick. A duct, which is often spoken of as the p.r.o.nephric duct (p.d.), and which we have figured under that name, is always developed. Anteriorly it opens into the body cavity. It is also called the Mullerian duct, and in the great majority of vertebrata it becomes the oviduct, uniting with its fellow, in the case of the dog-fish, ventral to the oesophagus. In the male it usually disappears; the uterus masculinus of the rabbit is still very generally regarded as a vestige of it. Kolliker has shown, however, that this interpretation is improbable. Ms. is the mesonephros, some or all of which becomes the epididymis in the male of types possessing that organ, and is connected with G. by the vasa efferentia. Mt., the metanephros, is, in -actual fact- [the frog], indistinguishably continuous with Ms., and is the functional kidney, its duct (metanephric duct) being either undifferentiated from the mesonephric (as is the case with the frog) or largely split off from it, as in the dog-fish, to form the ureter.

Section 20. The correspondence of the male organs of the dog-fish with those of the rabbit, will be more evident if the student imagine--

(a) the testes, vasa efferentia, and epididymis of each side to shift posteriorly until they reach a position on either side of the cloaca; and

(b) The uro-genital apertures, instead of meeting dorsally and posteriorly to the a.n.u.s, to shift round that opening and meet anteriorly and ventrically to it.

Section 21. This completes our survey of this type. Except where we have specified differences, the general plan of its anatomy follows the lines of the other vertebrate types described.

_Questions on the Dog-Fish_

1. Describe the alimentary ca.n.a.l of the dog-fish, and compare it with that of the rabbit in detail.

2. Compare the coelom of the dog-fish and rabbit.