Scyllium canicula.

I. EXOSKELETON.

The exoskeleton of the dogfish is mainly composed of placoid scales, each of which consists of a little bony base imbedded in the skin, bearing a small backwardly-directed spine formed of dentine capped with enamel. The scales are larger on the dorsal than on the ventral surface, and on the jaws they are specially large and regularly arranged in rows, there forming the teeth. The margins of the jaws or lips are without scales.

A second exoskeletal structure is found in the fins, all of which, both paired and unpaired, have, in addition to their cartilaginous endoskeleton, large numbers of long slender horny fibres, the fin-rays, which are of exoskeletal origin.

II. ENDOSKELETON.

The endoskeleton of the dogfish consists almost entirely of cartilage, which however may become calcified in places, e.g. the centrum of each vertebra is lined by a layer of calcified tissue.

The endoskeleton is divisible into an axial portion consisting of the vertebral column, skull, and skeleton of the median fins, and an appendicular portion consisting of the skeleton of the paired fins and their girdles.

1. The Axial Skeleton.

A. The Vertebral Column and Ribs.

The vertebral column consists of a series of some hundred and thirty vertebrae, each of which is united with its predecessor and successor in such a way as to allow a large amount of flexibility.

These vertebrae are developed round an unsegmented rod, the notochord, which forms the axial support of the embryo. The notochord remains continuous throughout the whole vertebral column, but is greatly constricted opposite the middle of each vertebra, and thus rendered moniliform. The vertebrae are divided into two groups, an anterior group of trunk vertebrae, and a posterior group of caudal or tail vertebrae.

A typical vertebra consists of a middle portion, the centrum, a dorsal portion, the dorsal or neural arch, which surrounds the spinal cord, and a ventral portion, the ventral or haemal arch, which similarly encloses a space.

The tail vertebrae of the dogfish have this typical arrangement, the trunk vertebrae have the haemal arches modified.

Each centrum is a short cylinder of cartilage surrounding an hourglass-shaped cavity occupied by the notochord. The neural arches are composed of three separate elements, the vertebral neural plates (basidorsalia), intervertebral neural plates (interdorsalia), and neural spines (supradorsalia).

The vertebral neural plates are in the adult fused with their respective centra, and are notched behind for the exit of the ventral (motor) roots of the spinal nerves. The intervertebral neural plates are polygonal pieces alternating with the vertebral neural plates; they are notched behind, but at a more dorsal level than are the vertebral neural plates, for the exit of the dorsal or sensory roots of the spinal nerves.

The neural spines are small patches of cartilage filling up the gaps between the dorsal ends of the neural plates.

The haemal arches (basiventralia) differ much in the trunk and tail portions of the vertebral column. In the trunk portion the centra are flattened below, and the two halves of the haemal arch diverge from one another as blunt ventri-lateral processes to which short cartilaginous rods, the ribs, are attached. Further back at about vertebra 37, the two halves of the haemal arch project downwards and meet forming a complete arch. Further back still, towards the hind end of the tail, the haemal arches bear median haemal spines (ventrispinalia).

B. The Skull.

The skull of the dogfish remains cartilaginous throughout the life of the animal, and has consequently a far more simple structure than have the skulls of higher animals, in which complication has been produced by the development of bone.

The skull consists of the following parts:—

(1) a dorsal portion, the cranium, which lodges the brain, and to the sides of which the capsules of the auditory and olfactory sense organs are united. The cranium may be compared to an unsegmented continuation of the vertebral column;

(2) a number of ventral structures, disconnected or only loosely connected with the cranium. These together constitute the visceral skeleton forming the jaws and supporting the gills.

(1) The Cranium.

The Cranium is an oblong box, with a flattened floor and a more irregular roof. Its sides are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits.

(a) On the dorsal surface of the cranium the following points should be noticed. First at the anterior end, the large thin-walled nasal or olfactory capsules (fig. 6, 1), each of which is drawn out into a narrow cartilaginous process.

The olfactory capsules have no ventral walls, and are separated from one another by the internasal septum, which is drawn out into a third slender process. These three processes together constitute the rostrum (fig. 6, 2).

Behind the olfactory capsules comes a large, nearly circular, hole, the anterior fontanelle, slightly behind which are the two ophthalmic foramina. The dorsal and ventral boundaries of the orbits are respectively formed by the prominent supra-orbital and suborbital ridges. Behind are the auditory capsules (fig. 6, 8), each of which is marked by a pair of prominent ridges, converging towards the middle line to a pair of apertures. These apertures communicate with two canals, the aqueductus vestibuli, which lead into the internal ear. The two ridges lodge respectively the anterior and posterior vertical semicircular canals of the ear.

(b) The principal structures to be noted in a side view of the cranium are contained in the orbit or eye-cavity. Near the base of the orbit at its anterior end is seen the small orbitonasal foramen (fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is the large ophthalmic foramen (fig. 6, 5) so prominent in a dorsal view of the skull; through it the ophthalmic branches of the fifth and seventh nerves pass. Slightly further back near the ventral surface is the large optic foramen (fig. 6, II.) for the passage of the second nerve. Vertically above the optic foramen, near the dorsal surface, is the very small foramen for the fourth nerve (fig. 6, IV.). Behind and a little above the optic foramen is another small aperture, the foramen for the third nerve. Behind and slightly below this is the large foramen for the sixth and main branches of the fifth and seventh nerves (fig. 6, V.). In front of and slightly below this foramen are seen two other small apertures; the more anterior and ventral of these (fig. 6, 4) is for the passage of a vessel connecting the efferent artery of the hyoid gill with the internal carotid artery inside the skull, the more posterior and dorsal is for the interorbital canal (fig. 6, 3) which unites the two orbital sinuses. Above and very slightly in front of the large foramen for the sixth and main parts of the fifth and seventh nerves, are two small foramina (fig. 6, Va., and VIIa.), through which the ophthalmic branches of the fifth and seventh nerves enter the orbit. Behind and slightly below the large foramen just mentioned is a small hole through which the external carotid enters the orbit (fig. 6, 9).

Behind the orbit is the auditory capsule. This is marked below by a prominent surface for the articulation of the hyomandibular, above which is the deep postorbital groove for the passage of a blood-vessel, connecting the orbital and anterior cardinal sinuses.

(c) Passing to the posterior end of the cranium: in the centre is seen the large foramen magnum through which the brain and spinal cord communicate. The notochord enters the skull just below this foramen, and on each side of the notochord is a projection, the occipital condyle, by which the first vertebra articulates with the skull.

External to the condyles are the prominent pneumogastric foramina for the passage of the tenth nerves, and further to the sides, just beyond the posterior vertical semicircular canals, are a pair of deep pits in which lie the foramina for the ninth nerves (fig. 6, IX).

(d) The broad and flat ventral surface of the cranium is continued in front as the internasal septum and terminated laterally by the suborbital ridges. At a little behind the middle it is traversed by two shallow grooves along which the internal carotid arteries run. At the divergent ends of these grooves are seen two small apertures through which the external carotids enter the orbit (fig. 6, 9), and at the point where they meet is a single small aperture through which the internal carotid enters the cranium.

(2) The Visceral Skeleton.

The Visceral skeleton forms a series of seven cartilaginous arches or hoops, surrounding the anterior part of the alimentary canal, and enclosing a wide but rather shallow space.

(a) The first or mandibular arch is the largest of the series, and forms the upper and lower jaws. Each half of the upper jaw or palato-pterygo-quadrate bar is formed by a thick cartilaginous rod which meets its fellow in the middle line in front, the two being united by ligament. Each half is connected to the cranium just in front of the orbit by the ethmo-palatine ligament (fig. 6, 10), and at its hind end articulates with one of the halves of the lower jaw. Each half of the lower jaw or Meckel's cartilage (fig. 6, 12) is a cartilaginous bar, wide behind but narrow in front, where it is united to its fellow by a median ligament. Imbedded in the tissue external to the upper jaw are a pair of labial cartilages, and a similar but smaller pair are imbedded in the tissue external to the lower jaw.

The jaws are developed from a structure whose dorsal and ventral portions subsequently become of very different importance. The ventral portion forms both upper and lower jaws, the former being developed as an outgrowth from the latter. The dorsal portion forms only the pre-spiracular ligament (fig. 6, 20), a strong fibrous band containing a nodule of cartilage, and running from the anterior part of the auditory capsule to the point where the jaws are connected with the hyomandibular.

(b) The hyoid arch consists of a pair of cartilaginous rods which are attached at their dorsal ends to the cranium, and are united ventrally by a broad median plate of cartilage, the basi-hyal. Each rod is divided into a dorsal portion, the hyomandibular and a ventral portion, the cerato-hyal. The hyomandibular (fig. 6, 13) is a short stout rod of cartilage projecting outwards, and somewhat backwards and downwards from the cranium, with which it articulates behind the orbit and below the postorbital groove. Its distal end articulates with a rather long slender bar, the cerato-hyal (fig. 6, 14), which is in its turn attached to the side of the basi-hyal. The basi-hyal is a broad plate, rounded in front and drawn out behind into two processes to which the two halves of the first branchial arch are attached. The posterior surfaces of both hyomandibular and cerato-hyal bear slender cartilaginous processes, the gill rays. The hyoid arch forms the main suspensorium or means by which the jaws are attached to the cranium. This attachment is chiefly brought about by a series of short ligaments which connect the posterior ends of both upper and lower jaws with the hyomandibular, but there is also a ligament connecting the lower jaw with the cerato-hyal. The attachment of the jaws to the cranium is also partially effected by the pre-spiracular and ethmo-palatine ligaments.

(c) Each of the five branchial arches is a hoop, incomplete above and formed of four or more pieces of cartilage. The most dorsal elements, the pharyngo-branchials, are flattened, pointed plates whose free inner ends run obliquely backwards, and terminate below the vertebral column. They are connected at their outer ends with the short broad epi-branchials (fig. 6, 16) which lie at the sides of the pharynx. From the epi-branchials arise the long cerato-branchials (fig. 6, 17) which run forwards and inwards along the ventral wall of the pharynx. The first four cerato-branchials are connected with small rods, the hypo-branchials, which run backwards to meet one another in the middle line. The last two pairs of hypo-branchials and the fifth cerato-branchials are connected with a broad median plate, the basibranchial. Along the outer sides of the second, third and fourth cerato-branchials are found elongated curved rods, the extra-branchials (fig. 6, 19). The epi-branchials and cerato-branchials bear gill rays along their posterior borders.

C. The Skeleton of the Median Fins.

The dorsal fins have a skeleton consisting of a series of short cartilaginous rods, the basals or basalia, which slope obliquely backwards. Their bases are imbedded in the muscles of the back, while their free ends bear a number of small polygonal cartilaginous plates, the radials or radiale. Associated with this cartilaginous skeleton are a number of long slender horny fibres, the fin-rays, which have been already referred to in connection with the exoskeleton. The skeleton of the other median fins mainly consists of these fibres, the cartilaginous portion being reduced or absent.

2. The Appendicular Skeleton.

This includes the skeleton of the two pairs of limbs and of their respective girdles.

The Pectoral girdle forms a crescent-shaped hoop of cartilage, incomplete above and lying just behind the visceral skeleton. The mid-ventral part of the hoop is the thinnest portion, and is drawn out in front into a short rounded process which is cupped dorsally and supports part of the floor of the pericardium (fig. 7, 1). On each side of this flattened mid-ventral portion the arch becomes very thick and bears on its outer border a surface with which the three basal cartilages of the fin articulate. The dorsal ends or scapular portions of the girdle form a pair of gradually tapering horns.

The Pectoral fin articulates with the pectoral girdle by means of three basalia or basal cartilages, the propterygium, meso-pterygium and meta-pterygium. The most anterior and the smallest of these is the propterygium (fig. 7, 5), while the most posterior one, the meta-pterygium (fig. 7, 3), is much the largest. Along the outer borders of the three basalia are arranged a series of close set cartilaginous pieces, the radiale. The propterygium supports only a single radial, which is however much larger than any of the others. The meso-pterygium also supports only a single radial which divides distally.

The meta-pterygium bears about twelve long narrow radials, the first nine of which are traversed by a transverse joint at about two-thirds of the way from their origin. Succeeding the radials are a series of small polygonal pieces of cartilage arranged in one or more rows and attached to the ends of the radials, and finally the fin is completed by the dermal fin-rays.

The Pelvic Girdle is much smaller than the pectoral. It is formed of a stout nearly straight bar of cartilage placed transversely across the ventral region of the body. The bar has no dorsal or lateral extensions, and is terminated by short blunt processes. It bears on its posterior surface a pair of facets with which the pelvic fins articulate.

The Pelvic Fin is smaller and more simply constructed than is the pectoral. It consists of a long, somewhat curved rod, the basi-pterygium (fig. 8, 2), running directly backwards on the inner side of the fin, and articulating in front with the pelvic girdle. From its outer side arise a series of about fourteen parallel cartilaginous radials which bear smaller polygonal pieces. The anterior one or two of these radials may articulate independently with the pelvic girdle. In the adult male dogfish the distal end of the basi-pterygium bears a stout rod nearly as long as itself, and grooved on the dorsal surface. This is the skeleton of the clasper (fig. 8, 3).