A Guide for the Dissection of the Dogfish (Squalus Acanthias)

by Lawrence Edmonds Griffin

The palato-quadrate and Meckelian cartilages are suspended from the hyomandibular by several strong ligaments, the direct attachments of the jaws to the cranium being of soft connective tissue only. Both the hyomandibular and ceratohyal cartilages bear slender rods (branchial rays) on their posterior edges, which support the anterior wall of the first gill pouch. Note the position of the spiracle between the mandibular and hyoid arches. The anterior wall of the spiracle is strengthened by two small, flat, vertical cartilages, probably homologous with the branchial rays of the gill arches.

The remaining five visceral arches differ little in their construction. Dorsally, each has a flat, sickle-shaped pharyngo-branchial cartilage attached to the vertebral column by fibrous bands. The pharyngo-branchials of the last two arches are fused. Ventrad to each pharyngo-branchial is an epibranchial cartilage. The next segment of each arch is formed by the ceratobranchial cartilage. All the epibranchials and ceratobranchials except those of the fifth arch bear slender branchial rays. The ventral ends of the ceratobranchials articulate with each other, the first being attached to the ceratohyal by ligament. The second, third, and fourth arches have another more ventral series of cartilages, the hypobranchials. The lower ends of the hypobranchials are attached to a large median plate, the basibranchial. The fourth ceratobranchial joins the third hypobranchial, while the ceratobranchials of the fifth arch are attached to the basibranchial directly. The basibranchial is composed of two segments closely united by ligament; the anterior one narrow, the posterior broad and flat in front, tapering to a sharp point behind.

Short teeth of cartilage, called gill rakers, project into the pharynx from the inner edges of the arches.

A dorsal and a ventral series of extra-branchial cartilages, thin, slender plates, lie on the external side of each gill arch.

Pectoral girdle and fin. Remove from the body the pectoral girdle, with the fins attached, and carefully scrape off the muscles from the cartilaginous parts. It will be found that the support of the fin is partly of cartilaginous plates and rods, partly of horny fibres (dermal fin-rays) which overlie the extremities of the cartilages and extend to the edges of the fin. These fibres are in two layers, one beneath the skin of each side. They are formed in the dermis. A similar arrangement of horny fibres is found in all the other fins.

The pectoral girdle passes across the ventral surface of the body and upward on each side to the level of the vertebral column. The stout ventral bar presents numerous facets for the origin and insertion of muscles. The articular surfaces for the pectoral fins are well up on the sides of the girdle. The slender dorsal end of each side of the girdle consists of a separate bar of cartilage, movably articulated to the lower portion. The ascending limb of the girdle, from the fin articulations to the base of the cartilage just mentioned, is called the scapular portion; the small bar is the supra-scapular; the ventral bar between the fin articulations is the coracoid portion.

The cartilaginous skeleton of the pectoral fin consists primarily of a row of three basal cartilages, all articulating proximally with the girdle. The middle basal is much the largest. Distal to the basals are three rows of rod-like radial cartilages, the proximal row being articulated to the basals.

Pelvic girdle and fin. Remove the pelvic girdle from the body with the pelvic fins attached, and clean away the muscles.

The pelvic girdle consists of an almost straight bar of cartilage, slightly thicker at its middle than at its ends, which lies transversely in the ventral wall of the abdomen. To each end is attached a long basal cartilage which lies in the fin, close to and parallel with its inner margin. A proximal series of slender radial cartilages is attached to the lateral side of the basal; a distal series of very short radials lies outside of the first series, while the portion of the fin beyond these is supported by the dermal fin-rays.

First dorsal fin. Remove the mass of muscles on both sides of the base of the fin down to the vertebral column. The principal cartilages of the fin lie in the median connective tissue septum which separates the dorsal musculature of the two sides of the body. The basal cartilages of the fin are attached to the vertebral column by means of this septum. It is best to remove the underlying portion of the column with the fin. The cartilages can then be scraped perfectly clean. The skeleton of the fin is composed of three rows of cartilages: (1) a basal row consisting of one very large, flat plate and two or three smaller ones posterior to it; (2) an intermediate row of several plates of nearly equal size; and (3) a distal row of several very small plates. The intermediate and distal rows extend beyond the body musculature into the base of the fin. The remainder of the fin is supported by the dermal rays. In front of the cartilages which have been mentioned is the strong spine of dentine (see p. 5), with its free portion sheathed by an enamel-like covering.