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.
| 1. hollow in the mid-ventral part | 5. propterygium. |
| of the pectoral girdle which | 6. propterygial radial. |
| supports the pericardium. | 7. meso-pterygial radial. |
| 2. dorsal (scapular portion) of | 8. meta-pterygial radial. |
| pectoral girdle. | 9. outline of the distal part of |
| 3. meta-pterygium. | the fin which is supported |
| 4. meso-pterygium. | by horny fin-rays. |
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.
| 1. pelvic girdle. | 3. clasper. |
| 2. basi-pterygium. | 4. radiale. |
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).
CHAPTER VII.
THE SKELETON OF THE CODFISH.[35] (Gadus morrhua.)
I. EXOSKELETON.
The exoskeleton includes
(1) Scales. These are of the type known as cycloid and consist of flat rounded plates composed of concentrically arranged laminae of calcified matter, with the posterior margin entire. The anterior end of each scale is imbedded in the skin and is overlapped by the preceding scales.
(2) The teeth. These are small, pointed, calcified structures arranged in large groups on the premaxillae, mandible, vomer, and superior and inferior pharyngeal bones.
(3) The fin-rays. These are delicate, nearly straight bony rods which support the fins.
II. ENDOSKELETON.
The endoskeleton of the Codfish, though partially cartilaginous, is mainly ossified.
It is divisible into an axial portion, including the skull, vertebral column, ribs, and skeleton of the median fins, and an appendicular portion, including the skeleton of the paired fins and their girdles.
1. The Axial Skeleton.
A. The Vertebral Column.
This consists of a series of some fifty-two vertebrae, all completely ossified.
It is divisible into two regions only, viz. the trunk region, the vertebrae of which bear movable ribs, and the caudal or tail region, the vertebrae of which do not bear movable ribs.
Trunk vertebrae.
These are seventeen in number; the ninth may be described as typical of them all. It consists of a short deeply biconcave centrum whose two cavities communicate by a narrow central canal. From the dorsal surface of the anterior half of the