Vertebrate Zoology G. R. De Beer (1928)

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Chapter IV Scyllium, A Chordate with Jaws, Stomach, and Fins

Externals

The dogfish possesses an elongated body with a distinct head and tail, the latter provided with a tail-fin of which the ventral lobe is larger than the dorsal (heterocercal). There are two median dorsal fins. The most obvious advance over the Cyclostome condition is the possession of paired fins, of which there are two pairs : a pectoral and a pelvic.

The head has paired nasal sacs and eyes. The mouth is situated some distance behind the anterior end of the snout. Behind the mouth on each side are six openings into the pharynx. The first pair of these is small and more dorsally situated than the others ; it is the spiracle. The remaining five are the gill-slits, numbered i to 5. The anus lies in a cloaca (joint opening of the alimentary and urino-genital systems) in the midventral line behind the pelvic fins, and on each side of it is a small pore (the so-called abdominal pore) communicating with the coelom. In the male, there is a pair of claspers on each side of the cloaca.

Denticles

The body is covered all over with small sharp spikes, with the points directed backwards. These are the placoid scales or denticles. They are made of dentine covered over with a cap of enamel. Dentine is a hard substance produced by mesodermal cells beneath the epidermis ; and it is identical with the substance of which the teeth of all vertebrates are made. It consists of a calcified ground-substance in which filamentous processes of cells are to be found, but no cells themselves. In this particular it differs from bone. The enamel is formed from the ectoderm. On the inner rim of the jaws just inside the mouth, denticles are also found. They get pushed out to the edge of the jaws, and act as biting teeth. When worn out, they are replaced by others which are pushed up in their turn to the biting edge (see Fig. 122, p. 263).

Lateral Line

In various places over the body, there are peculiar organs belonging to what is known as the lateral-line system. Essentially, they take the form of canals sunk beneath the skin, and opening to the exterior at intervals. In these canals are sense organs whose probable function it is to appreciate low-frequency vibrations in the water. One of these canals runs along the side of the body from the tail to the head, and is the true " lateral-line canal." At about the level of the first gill-slit, it gives off a transverse occipital canal which runs over the top of the head and meets its fellow from the opposite side. It continues forwards over the spiracle as a short temporal or postorbital canal, and divides into two. One portion goes forwards over the eye as the supraorbital canal, the other beneath it as the infraorbital canal. In addition there is typically a canal which runs down behind the spiracle (hyomandibular canal) to the lower jaw (mandibular canal) ; but these two are more or less interrupted in the dogfish.

Close to the lateral-line canals in various places there are little pits leading from pores on the surface down narrow tubes to ampullar at the bottom , where there are sense-organs . These are the pit-organs, or ampullae of Lorenzini. Together with the lateral-line system they constitute the neuromast, or acustico-lateralis organs, to which the ear also belongs. These organs are also present in Petromyzon, but not so definitely arranged.

Fig. 20. — Scyllium : section through the skin showing the ampullar of Lorenzini (aL), denticles (d), nerve-fibres (w), opening of a lateral- line canal (oe), and a sense-organ (so) in the canal.

Ear

The ear consists of a pit sunk in from the skin and forming a sac, which communicates with the exterior by a long tube and a fine pore ; the ductus endolymphaticus. The sac is divided into a more dorsal utricle, and a more ventral saccule.

The utricle bears three semicircular canals, at the base of each of which is a swelling or ampulla containing an organ of balance. The whole ear is to be regarded as a very much enlarged lateral-line organ.

Eye

The eyes are hollow cups with sensitive retinal layer, iris, pigment layer, vascular choroid, and protective cartilaginous sclerotic. Fitting into the aperture of the eye-cup is the spherical lens, which is attached to the cup by a ventral ciliary muscle.

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Fig, 21. — Scyllium : view of the outer side of the left auditory sac.

a, ampulla ; 06, 2c, and /v. anterior, lateral, and posterior semicircular canals ; Jc. ductus endolymphaticus : s, saccule ; If, utricle.

The movements of the eyeball are effected by six muscles. Four oi these (superior, internal, inferior, and external rectus muscles') exert straight pulls on the four cardinal points of the eyeball, and turn it upwards, forwards, downwards, or backwards respectively. The two others (superior and inferior oblique muscles) pull it obliquely either forwards and upwards or forwards and downwards.

Nose

The nose is formed by a pair of pits on each side of the under surface of the snout just in front of the mouth, and connected with it by grooves which run to its corners. Inside the pits, the sensory olfactory epithelium is thrown into a number of folds.

Nervous System

As in the Cyclostome, the brain is divisible into fore-, mid-, and hind-regions. Further, the fore- and hind-brains can also be divided into two for facilitating description. There are therefore five sections of the brain, whose names from front to rear are : telencephalon, diencephalon (also called thalamencephalon), mesencephalon, metencephalon, and myelencephalon. The first two divisions together form the forebrain or prosencephalon, the last two form the hindbrain or rhombencephalon. The sides of the telencephalon (or end-brain) are greatly expanded, and bear the olfactory bulbs. On the floor is the optic chiasma, where the optic nerves cross over from one side to the other. In front of this is the lamina terminalis ; the thickened lower portions of the side walls are the corpora striata.

A transverse fold in the roof, the velum transversum, marks the beginning of the diencephalon (or between-brain). The sides are thickened and known as the optic thalami, the floor is depressed to form the infundibulum to which the pituitary is attached. The roof bears a projection : the epiphysis, vestige of the pineal eye.

The floor, sides, and roof of the mesencephalon are thickened, so that its cavity is reduced and is known as the aqueduct of Sylvius. The roof forms the paired optic lobes.

The roof of the metencephalon is thick and forms the cerebellum, that of the myelencephalon is thin. To the sides of and behind the cerebellum are the restiform bodies.

The cavity of the forebrain is called the 3rd ventricle ; that of the hindbrain the 4th ventricle. The brain is surrounded by a membrane carrying blood-vessels (the pia mater), and this dips down in folds from the roof of the 3rd and of the 4th ventricles to form a choroid plexus. Connecting one side of the brain with the other there are tracts of fibres called commissures. Of these, the habenular and the posterior are in the roof of the between-brain and midbrain respectively

There is also an (anterior) transverse commissure in the lamina terminalis, but on the whole there is little interconnexion between the two sides of the brain.

The myelencephalon, or medulla oblongata, passes back gradually into the spinal cord. This is a tube with thick walls and a small central cavity, continuous of course with that of the brain. The nerve-cells are grouped round the centre of the cord, and form the " grey matter." Outside them and occupying the remaining space are the ascending and descending tracts of nerve-fibres, provided with medullary sheaths, and forming the " white matter." This arrangement of central grey matter and peripheral white matter holds also in the brain. Only in the cerebellum and in the optic lobes are there some superficial nerve-cells ; i.e. grey matter outside white.

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Fig. 22. — Scyllium : median view of a longitudinal section through the brain.

The various regions of the brain are separated by broken lines across the central cavity, and indicated by the letters : T, telencephalon (end- brain) ; D, diencephalon (or thalamencephalon, between-brain) ; Ms, mesencephalon (midbrain) ; Mt, metencephalon (anterior part of hind- brain) ; My, myelencephalon (posterior part of hindbrain) ; ac, anterior commissure ; c, cerebellum ; cp 3 and 4, choroid plexus of the third and fourth ventricle ; he, habenular commissure ; i, infundibulum ; //, lamina terminalis ; mo, medulla oblongata ; oc, optic chiasma ; ol, olfactory lobe ; op, optic lobe ; pb, pituitary body ; pc, posterior commissure ; pe, pineal stalk ; Rf, Reissner's fibre ; rn, recessus neuroporis ; sc, spinal cord ; v 3 and 4, cavity of the third and fourth ventricle ; vt, velum transversum. (Partly after Nicholls.)

Attention may here be called to Reissner's fibre. It is a wire-like structure of unknown function which runs from the posterior commissure in the roof of the midbrain, through the cavity of the nerve-tube right down to its hind end where it is attached. Reissner's fibre is present in most chordates, but not in Amphioxus nor in man.

The pia mater has already been mentioned. It encircles

Fig. 23. — Scyllium : ventral view of the brain showing the pituitary body.

al, anterior lobe ; nil, neuro-intermediate lobe ; vl, ventral lobe of the pituitary body ; il, floor of the diencephalon ; on, optic nerve ; sv, saccus vasculosus.

the whole nerve-tube, and corresponds to the choroid layer of the eye. Outside it is a tougher membrane, the dura mater, protective in function. It is applied to the inner wall of the skull and corresponds to the sclerotic layer of the eye.

Spinal Nerves

In each segment on each side, behind the head, the spinal cord gives off a ventral nerve, and a dorsal nerve with a ganglion on it. These two nerves join to form a mixed spinal nerve. Soon after joining, their components separate out again to their various destinations. The ventral roots are distributed to the muscles of the myotomes along the trunk and in the fins, the dorsal roots to the sense-organs. In addition, each spinal nerve sends a branch to the sympathetic ganglia, which are joined to one another by nerve-fibres which form two longitudinal chains, one on each side of the dorsal aorta.

The nerve-fibres are medullated, except those of the sympathetic system.

Cranial Nerves and Head-Segmentation. — The cranial nerves are of importance in unravelling the segmentation of the head. Some of them are dorsal roots, and some are ventral, but they never join to form mixed nerves as in the region of the trunk.

The olfactory nerve (No. I) is not a true nerve like the others, for it is formed of the fibres produced by the cells of the nasal epithelium which grow in to the forebrain (in the manner characteristic of all the nerves in Amphioxus). The small nervus terminalis which accompanies it for some distance is also devoid of segmental significance.

Similarly the optic nerve (No. II) is not segmental, for the whole optic cup and stalk are really parts of the brain itself.

The dorsal root of the first (or premandibular) segment is the profundus (No. V i), which unfortunately disappears in Scyllium. In the closely related Squalus it is present, and runs forwards through the socket for the eye (the orbit) under the superior oblique muscle, and innervates the skin of the snout.

Fig. 24. — Scyllium : dorsal view of a dissection of the cranial nerves, from a drawing by Mr. B. W. Tucker.

II, optic ; III, oculomotor ; IV, trochlear ; V md, mandibular branch of trigeminal ; V and VII op, superficial ophthalmic branches of trigeminal and facial ; V mx and VII b, maxillary branch of trigeminal and buccal branch of facial ; VII p, palatine branch of facial ; VII h, hyomandibular branch of facial ; VIII, auditory ; IX, glossopharyngeal ; X, vagus ; X b 2, second branchial branch of vagus ; X /, lateral-line branch of vagus ; X v, visceral branch of vagus ; ac, auditory capsule ; c, cerebellum ; e y eye ; er y external rectus eye-muscle ; gs 1 and 5, first and fifth gill-slits ; h, hypoglossal nerve ; io, inferior oblique eye-muscle ; ir, inferior rectus eye-muscle ; ol, olfactory lobe ; s, spiracle ; sc, spinal cord ; so, superior oblique eye-muscle ; sr y superior rectus eye-muscle ; t, terminalis nerve.

The corresponding ventral root of the first segment is the oculomotor (No. Ill), which supplies the following four eye- muscles : superior, internal, and inferior rectus, and the inferior oblique. It is also connected with the sympathetic ciliary ganglion.

The second or mandibular segment has as its dorsal root the trigeminal (No. V, 2 and 3). This nerve is composed of a superficial ophthalmic branch running forwards over the eye, a maxillary branch in the upper jaw and a mandibular branch in the lower jaw. These nerves are distributed to sense- organs in the skin, and also to the muscles which move the jaws.

The corresponding ventral root of the second segment is the trochlear (also called pathetic ; No. IV), which innervates the superior oblique eye-muscle.

The dorsal root of the third or hyoid segment is the facial nerve (No. VII). It is made up of the following branches : —

superficial ophthalmic, running forwards over the eye in company with that of the trigeminal, and innervating the supraorbital lateral-line organs ;

buccal, running forwards beneath the eye and innervating the infraorbital lateral-line organs ;

hyomandibular, passing down behind the spiracle to innervate the lateral- line organs of the lower jaw, and the muscles of the hyoid arch ;

palatine, innervating the taste-organs on the roof of the mouth ;

pretrematic, running down in front of the spiracle, innervating sense-organs.

The auditory nerve (No. VIII), which innervates the sense- organs of the ear, is really an enlarged and specialised branch of the facial nerve.

The corresponding ventral root of the third segment is the abducens (No. VI), which supplies the external rectus eye- muscle.

The dorsal root of the fourth segment is the glossopharyngeal (No. IX). It has a branch to the temporal region of the lateral-line canal, a pharyngeal branch to the gut, and a branchial branch which divides ; a small branch running in the hyoid arch in front of the first gill-slit, and the main branch running behind the ist gill-slit in the ist branchial arch. The glossopharyngeal thus bears the same relations to the ist gill-slit as the facial does to the spiracle.

There is no ventral root to the fourth segment ; the somite which it would innervate disappears.

The fifth segment has also lost its myotome and ventral root during development. To each of the remaining gill- slits, 2nd to 5th, there corresponds a branchial nerve, the main branches running behind the slits and pretrematic branches in front of them. These nerves are the dorsal roots of the fifth, sixth, seventh, and eighth segments, which have joined together to form the vagus (No. X). The lateral-line organs in the occipital region of the head and all along the side of the trunk to the tail, are innervated by branches of the vagus. In addition, the vagus sends a visceral branch to the heart and stomach, forming part of the parasympathetic system.

The ventral roots of the sixth and following segments innervate the myotomes of their segments, and also contribute to a nerve — the hypoglossal — which runs back over the gill- slits, down behind them and forwards beneath them to innervate some muscles under the pharynx.

The ninth is the first segment to have a fully formed mixed spinal nerve.

Skull

The skull and all the skeleton is made of cartilage.

The glossopharyngeal and vagus nerves emerge well in front of the hind end of the skull. The latter therefore occupies a larger number of segments than in Petromyzon, namely seven. The skull encloses the brain completely except for an aperture in its roof. The cranial nerves all emerge through special holes or foramina. The auditory and olfactory capsules are firmly fused on. The notochord disappears in the skull-region, and a definite joint is formed between the hind end of the skull and the front of the vertebral column.

The jaws are formed by the skeleton of the first or mandibular visceral arch, which separates the mouth from the spiracle. The upper and lower portions of this arch's skeleton move on one another and form the upper and lower jaws. The skeleton of the upper jaw is the ptery goquadrate, that of the lower is Meckel's cartilage. The possession of jaws is the criterion of the group Gnathostomata, to which Scyllium and all higher forms belong. The arches between the gill-slits also have cartilaginous rods. That of the second or hyoid visceral arch (separating the spiracle from the 1st gill-slit) is composed of a dorsal portion, the hyomandibula ; and a ventral portion, the ceratohyal, and basihyal. The following visceral arches are made up of four pieces on each side, which are from above downwards, the pharyngobranchial, epi-, cerato-, and hypobranchial. There is also a median basi- branchial. The pterygo-quadrate and the hyomandibula represent the " epi " elements of their respective arches, Meckel's cartilage and ceratohyal the There is no difficulty in recognising the fact that the jaws are simply slightly modified visceral arches. These cartilaginous arches lie in the splanchnopleur. Stiffening the partitions between the gill-slits are extrabranchials and branchial rays.

Fig. 25. Scyllium : view of skull and visceral arches. a, foramen for efferent pseudobranchial artery ; ac, auditory capsule ; c, centrum of vertebra ; ch, ceratohyal ; cb 2, ceratobranchial of second arch ; d, foramen for dorsal spinal nerve-root ; eb 4, epibranchial of fourth arch ; hb 2, hypobranchial of second arch ; hm, hyomandibula ; id, interdorsal cartilage ; Mc, Meckel's cartilage ; o, orbit ; oc, olfactory capsule ; pb 1 , pharyngobranchial of first arch ; pg, pterygoquadrate cartilage ; v, foramen for pituitary vein ; vr, foramen for ventral spinal nerve-root ; II, optic nerve foramen ; III, oculomotor nerve foramen ; IV, trochlear nerve foramen ; V and VII, trigeminal and facial foramen ; V and VII o, foramina for ophthalmic branches of trigeminal and facial.

It is important to notice that these jaws and branchial arches, which together constitute the splanchnocranium, are not fused on to the neurocranium or attached to it otherwise than by ligaments.

The pterygo-quadrate is slung from the skull by the hyomandibula, the upper end of which is attached to the auditory capsule. This method of suspension of the upper jaw is called hyostylic. The upper jaw does not touch the neurocranium itself. In addition to its attachment by the hyomandibula, there are two ligaments, the ethmoid and the postspiracular, which tie the upper jaw to the brain-case.

The ordinal numbers by which the arches, segments, and slits are known are unfortunately liable to lead to confusion, for which reason they are tabulated below : Slits or Clefts.

i st visceral = spiracle 2nd visceral = i st gill slit 3rd visceral = 2nd gill slit 4th visceral =3rd gill slit 5th visceral =4th gill slit 6th visceral = 5th gill slit Vertebral Column. — Corresponding to each septum between two segments, there are paired basidorsal and basiventral cartilages, surrounding the notochord. The sheath of the notochord is penetrated by these cartilages which, together, form a bobbin-like ring or centrum, which constricts and interrupts the notochord. The centra articulate on one another end to end, and in this way a vertebral column is formed. Rising up from the centra are the neural arches, which enclose the spinal cord in a canal. Alternating with these are interdorsal cartilages. The ventral nerves emerge behind the neural arches, and the dorsal roots behind the interdorsal cartilages. Ventral extensions of the basiventrals beneath the centra in the tail-region form haemal arches, in which blood-vessels run. Lateral extensions of the basiventrals give rise to the ribs. They extend in the septum that divides the myotomes horizontally, and are called " dorsal " ribs (see p. 307).

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Segments. 1 st = premandibular 2nd = mandibular

Arches or Bars. 1st visceral = mandibular

3rd=hyoid

2nd visceral =hyoid

4th

3rd visceral = 1 st branchial

5th

4th visceral = 2nd branchial

6th

5th visceral = 3rd branchial

7th

6th visceral = 4th branchial

8th

7th visceral = 5 th branchial

Fig. 26. — Scyllium : ventral view of, A, pectoral, and B, pelvic girdle. by basipterygium ; c, coracoid region ; d, skeleton of clasper (present in the male) ; ms, mesopterygium ; mt , metapterygium ; n, nerve foramen ; p> pelvic cartilage ; pr, propterygium ; r, radials ; s, scapular region.

Fins

The median fins are supported by jointed cartilaginous rods or radials. These were originally direct continuations of the neural and haemal arches, but as a result of the shortening of the bases of the fins (or concentration), the radials no longer correspond with the vertebras, except in the ventral lobe of the tail. In addition to the cartilaginous radials, the web of the fin is supported by horny dermal fin- rays, close under the skin on each side of the radials. These rays, or ceratotrichia, are more numerous than the radials.

The paired fins also have an internal skeleton of cartilaginous radials, and are anchored to the body by girdles lying in the body- wall. The pectoral girdle is a half-hoop of cartilage set transversely to the long axis of the body, with the free ends pointing upwards. On each side is a hollow, the glenoid cavity, into which the cartilages of the fins fit. The latter cartilages are the most proximal radials, which form three large cartilages, the pro-, meso-, and metapterygia. The ventral portion of the pectoral girdle is termed the coracoid region ; from the glenoid cavity to the free tips which project dorsally, the cartilage is known as the scapular region.

The pelvic girdle is formed by a transverse cartilage, at each end of which an elongated backwardly-directed basi- pterygium is articulated. This basipterygium forms the axis of the pelvic fin, and bears a number of cartilaginous radials on its anterior border.

The pectoral and pelvic fins, as well as the median fins, have their webs supported by horny dermal fin-rays, the ceratotrichia.

Alimentary System

The mouth leads into the pharynx, the sides of which are pierced by the spiracle and the gill-slits. The food consists of fair-sized pieces of prey, seized by the jaws, and in no danger of being lost through the gill-slits. Behind the pharynx is the oesophagus which leads into a large stomach. In its formation, the gut has been kinked to the left, so that the stomach is a well-defined region. Ventral to it is a large liver with a gall-bladder from which a bile-duct leads to the intestine. In the U-shaped bend which the stomach makes with the intestine lies the pancreas, the duct from which enters the intestine close to the bile-duct. The intestine bends backwards and runs straight to the rectum, which has a small diverticulum (the rectal gland), and leads to the cloaca. The gut is considerably longer than the distance from the mouth to the cloaca, and the " slack " is accounted for by the asymmetry of the stomach. This asymmetry persists through all the higher vertebrates. The internal surface of the intestine is increased by a fold forming the spiral valve.

Coelom and Myotomes

The gut is suspended in the splanchnocoel by a dorsal mesentery. Anteriorly the splanchnocoel is almost completely cut off from the cavity of the pericardium by the transverse septum, which leaves only small pericardio-peritoneal canals. Posteriorly, the splanchnocoel is in communication with the exterior by the pair of abdominal pores.

The series of somites is not complete. The first three give rise to the eye-muscles, but the myotomes of the fourth and fifth segments disappear during development, leaving the sixth to form the first complete myotome. Each myotome is divided into a dorsal and a ventral portion by a horizontal septum. Into this septum the ribs (known as dorsal ribs) extend. The ventral portions of the most anterior myotomes send muscles forwards beneath the pharynx, in the midventral line. These hypoglossal muscles lose connexion with their original myotomes, and connect the ventral ends of the skeleton of the visceral arches with the coracoid region of the pectoral girdle. The fins contain muscles attached to the radials. These muscles are derived from the myotomes. All muscles derived from myotomes are striated, voluntary, and innervated by ventral nerve-roots. The muscles of the jaws and branchial arches, although visceral, are striated and voluntary ; they are, however, not innervated by ventral roots, but are supplied by dorsal cranial nerves. The remainder of the visceral muscles are all smooth and involuntary, and are to be found in the walls of the gut, blood-vessels, and oviducts. They are innervated by the autonomic system (sympathetic and parasympathetic).

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Fig. 27. — Scyllium : ventral view of dissection showing the alimentary system and the afferent branchial vessels (male). ab 1 and 5, first and fifth afferent branchial artery ; bd, bile-duct ; cl t clasper (present in the male) ; /, fold overlying groove running from the nasal sac to the mouth ; gi, first gill-slit ; 1, intestine ; /, liver ; m, mouth ; n, nasal sac ; p y pericardium ; pg, pectoral girdle cut ; pn, pancreas ; pv, hepatic portal vein ; r, rectum ; rg, rectal gland ; s, stomach ; sp, spleen ; sv, sinus venosus ; t, testis ; th, thyroid gland ; ug, urinogenital papilla ; v, ventricle of heart ; va, ventral aorta.

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Fig. 28. — Structure and relations of kidneys and ducts in Gnathostomes.

A, larval condition with a Miillerian duct (Md) as well as a Wolffian duct (Wd). The Bowman's capsules (Be) of the mesonephros communicate with the splanchnocoel (sc) through the mesonephric funnels (mf). i, intestine. B, condition in the adult female. The Miillerian duct persists and functions as an oviduct. The eggs freed from the ovary (0) enter the mouth of the oviduct (oM), and go down it, past the oviducal gland (og) where the shell is secreted. The Wolffian duct is purely excretory in function. C, typical condition of the adult male . The Miillerian duct has disappeared except for vestiges of its opening (rM), and the sperm-sac (ss). Sperms pass from the testis (t) through the vasa efferentia (ve) corresponding to mesonephric funnels, to the Wolffian duct. The latter is not only excretory but also genital in function , and is also called the vas deferens . D , condition of the adult male Scyllium. The Wolffian duct connects as usual with the anterior mesonephric tubules and vasa efferentia, but the more posterior part of the mesonephros (e) is solely excretory in function ; its tubules run into a collecting duct (cd) which has separated off from the Wolffian duct. The base of the latter or vas deferens is thickened to form the vesicula seminalis (vs), and it and the collecting duct open into the sperm-sac.

Urino-genital System

The kidney of the adult dogfish is a mesonephros, similar to that of Petromyzon. Here, however, the excretory and genital systems are closely associated, and it is necessary to treat them together. In Scyllium and all Gnathostomes, in place of the single mesonephric duct on each side, there are typically two. One of these, the Wolffian duct, can be regarded as the original mesonephric duct, and it continues to receive the tubules from the Bowman's capsules. The other is the Mullerian duct which opens into the coelom by the conjoined openings of the degenerated pronephric tubules, and leads straight back to the cloaca without any connexion with the mesonephric tubules. The degree of development which these ducts show depends on the sex of the animal.

In the male, the testes are paired, and are connected by their anterior ends to the mesonephric tubules by means of the vasa efferentia. These correspond to the original ccelomostomes. Through them the sperms reach the Wolffian duct, which becomes known as the vas deferens ; its posterior end swells to form the seminal vesicle. The anterior portion of the mesonephros therefore is concerned with the evacuation of the genital products in the male. The posterior portion (sometimes and incorrectly called the metanephros) is solely excretory in function. Its tubules run into a collecting duct which connects with the Wolffian duct, both running into a sperm-sac. The two sperm-sacs, one on each side, join to form a urino-genital sinus which opens into the cloaca by a urino-genital papilla. The Mullerian ducts in the male are reduced to a pair of funnels on the ventral side of the oesophagus, and the sperm-sacs.

In the female the mesonephros is entirely excretory in function. The Wolffian ducts are swollen posteriorly to form urinary sinuses which open to the cloaca by a urinary papilla.

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Fig. 29. — Scyllium : ventral view of a dissection of the urinogenital system of a male adult.

cd, collecting duct ; cl, clasper ; ek, excretory portion of the mesonephros ; ^alimentary canal cut ; p, pericardium ; pp, pericardio-peritoneal canal indicated by an arrow ; ri, rudimentary opening of the Miillerian ducts ; ss, sperm-sac ; t, testis ; ng, urinogenital papilla ; ve, vasa efferentia ; vs, vesicula seminalis ; Wd, Wolffian duct or vas deferens.

The right ovary hangs in the ccelom covered by a fold of coelomic epithelium ; the left ovary disappears. The large eggs when they are ripe drop free into the ccelom. They enter the funnel-shaped openings of the Mullerian ducts on the ventral side of the oesophagus, and pass down them. These

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Fig. 30. — Scyllium : ventral view of a dissection of the urinogenital system of a female adult.

ab, abdominal pore through which an arrow is passed ; t, oesophagus ; io, internal opening of the Mullerian ducts ; k, mesonephros ; Md, Mullerian duct ; o, ovary ; og, oviducal gland ; 00, opening of Mullerian duct into the cloaca ; p, pericardium ; pp, pericardio-peritoneal canal through which an arrow is passed ; r, rectum ; so, sinus venosus cut ; up, urinary papilla ; Wd, Wolffian duct.

ducts or oviducts swell out into the oviducal glands, where the horny egg-case is secreted, and open separately into the cloaca.

The Miillerian duct is always genital, the Wolffian duct is always excretory, but in the male it is genital in function as well. In some dogfish the mesonephric tubules retain their funnels, opening into the splanchnoccel.

Vascular System

The vascular system is built on the same plan as that of Petromyzon. The subintestinal vein, which forms the hepatic portal vein, runs to the liver in a portion of mesentery in company with the bile-duct. The veins in the body-wall (or somatic veins) consist of a pair of cardinal veins running parallel with and on each side of the dorsal aorta. They connect with the sinus venosus of the heart by the ductus Cuvieri, which cross the ccelom from the body- wall to the gut- wall, in the transverse septum. The anterior cardinals bring the blood back from the head (orbital sinus and jugular), and from the ductus Cuvieri backwards the veins are known as posterior cardinals. In addition there are paired inferior jugular sinuses bringing blood back from the ventral regions of the head, and paired lateral abdominal veins draining the ventral posterior regions of the body- wall. All these lead into the ductus Cuvieri. The hyoid sinus is in the hyoid arch.

The heart in its pericardium is bent on itself, and is in the form of an S. The sinus venosus, which receives the ductus Cuvieri and the hepatic sinus from the liver, opens into the auricle whence the blood passes through an opening guarded by valves to the thick- walled ventricle. This lies beneath and behind the auricle. In front of the ventricle is a muscular conus arteriosus with two rows of valves which prevent the blood from flowing back into the ventricle. The conus leads through a bulbus (see p. 330) to the ventral aorta from which five pairs of afferent branchial arteries are given off. These break down into the capillaries of the gill-lamellae in the hyoid and four branchial arches. Each set of lamellae on one wall of a slit is called a demibranch. There are two demibranchs in each gill-slit except the last, which has only an anterior one.

The oxygenated blood is collected up into four efferent branchial arteries which correspond to gill-slits 1 to 4. They lead to the median dorsal aorta. Each efferent branchial artery is made up of two collecting vessels one on each side of a gill-slit and joining above it. Each gill- arch therefore contains two such vessels and one afferent vessel. The demib ranch of the 5th gill- slit is drained by the 4th efferent branchial artery. The demibranch of the spiracle receives already oxygenated blood from the anterior demibranch of the 1st gill-slit. Since it does not oxygenate this blood itself it is called a pseu dob ranch. The vessel leading from it to the internal carotid inside the skull is the efferent pseudobranchial artery, which can be seen at the back of the orbit or eye-socket.

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Fig. 31 . Scyllium ; diagrammatic view of the venous system. ac, anterior cardinal ; bv, brachial vein ; c, caudal vein ; dC, ductus Cuvieri ; hp, hepatic portal vein ; hs, hyoid sinus ; hv, hepatic vein ; ij, inferior jugular sinus ; iv, iliac vein ; k, mesonephnc kidney ; I, liver ; la, lateral abdominal vein ; Ic, lateral cutaneous vein ; os, orbital sinus ; pc, posterior cardinal ; pv, pituitary vein ; rp, renal portal vein ; sv, sinus venosus ; v , ventricle of the heart.

The anterior prolongation of the dorsal aorta is the internal carotid artery which pierces the base of the skull and supplies the brain. The dorsal aorta gives off paired subclavian arteries to the pectoral fins. Into the mesentery suspending the gut it sends the following : cceliac, lieno-gastric, anterior and posterior mesenteric arteries, which between them vascularise the stomach, liver, intestine, and the well-developed spleen. Farther back the dorsal aorta gives off pelvic arteries to the pelvic fins and renal arteries to the kidneys as well as branches to the gonads, and continues into the tail as the caudal artery. The blood from the tail is returned forwards by the caudal vein which forks, one branch going to each kidney as the renal portal vein. From there the blood is removed by the posterior cardinals.

Ductless Glands

(see Chapter XXXIII). — The spleen has already been mentioned ; it lies in the mesentery near the stomach (see p. 152).

With the development of jaws, the ciliary mode of feeding has been abandoned. The wheel- organ of Amphioxus is represented by an ectodermal ingrowth — the hypophysis, which comes into intimate contact with the infundibulum of the brain to form the pituitary body.

Fig. 32. — Scyllium : diagrammatic view of the arterial system, and of the sympathetic, supra-renals, and inter-renal (female specimen). Partly from a drawing by Mr. J. Z. Young.

c, caudal artery ; Ca, cceliac artery ; da, dorsal aorta ; eb 2, second efferent branchial artery; ep, efferent pseudobranchial artery; gi, first gill-slit ; ia, iliac artery ; tc, internal carotid artery ; ir, inter-renal body ; k, mesonephric kidney ; s, spiracle ; sa, subclavian artery ; sg, sympathetic ganglia, some of which are interconnected by the longitudinal sympathetic nerve-chains ; sr, supra-renal bodies.

The endostyle is not present as such, but it has been transformed into the thyroid gland, situated beneath the floor of the pharynx, as in the adult Petromyzon. In some dogfish, its cells still bear cilia.

The lining of the top of the gill-slits grows upwards to form a number of paired glandular masses ; in close association with the anterior cardinal veins they form the thymus glands. Lying on the course of the sympathetic nerve-chains, there are a number of bodies of the same origin and nature as the sympathetic ganglion-cells. They are the supra-renal bodies and originate from the nerve-tube. Posteriorly, between the kidneys is an elongated structure, the inter-renal, which is formed from the coelomic epithelium. It is interesting to find these two sets of structures separate, for in higher forms they combine to form the adrenals.

Characters present in Scyllium, which are lacking in Petromyzon : Biting jaws ; Paired fins ; Denticles (" placoid scales ") ; Dermal fin-rays (ceratotrichia) ; Definite stomach and pancreas ; Mixed spinal nerves (dorsal and ventral roots joined) ; Vertebral column constricting the notochord ; Dorsal ribs ; Myotomes separated into dorsal and ventral portions ; Miillerian and Wolffian ducts ; Seven segments included in the skull.

Characters of Scyllium which are primitive when compared with higher forms : Absence of bone ; Absence of swim-bladder ; Gill-slits opening separately to the outside ; Heart with single auricle and single ventricle ; Separate supra-renals and interrenals.

Literature

Bourne, G. C. An Introduction to the Study of the Comparative Anatomy of Animals. Vol. 2. Bell, London, 191 5.

Cahn, A. R. The Spiny Dogfish. A Laboratory Guide. Macmillan, New York, 1926.

Daniel, J. F. The Elasmobranch Fishes. University of California Press, 1922.

Goodrich, E. S Vertebrata Craniata. Cyclostomes and Fishes. Black, London, 1909.

Nicholls, G. F. The Structure and Development of Reissner's Fibre and the Subcommissural Organ. Quarterly Journal of Microscopical Science. Vol. 58. 1913.

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