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

by Lawrence Edmonds Griffin

In Eugaleus there is a fold of skin stretched across the lower part of the eye which serves as an eye-lid, and corresponds to the so-called “third eye-lid” or nictitating membrane of other vertebrates.

On each side of the neck are five vertical gill-clefts, each leading into a large gill pouch which communicates with the pharynx by an internal opening. Pass a probe through a gill-cleft into the mouth.

Back of each eye is a small aperture, the spiracle; explore this cavity with a probe. The spiracle is to be considered a gill-cleft moved forward upon the head and largely, though not entirely, deprived of its respiratory functions.

In the center of the dorsal surface of the head, between the spiracles, are two pores, the external openings of the endolymphatic ducts which communicate with the internal ear. Large numbers of smaller pores can be found on all surfaces of the head, some in groups, some arranged linearly, many scattered. Most of those arranged in lines lead into the sensory canal system which continues from the lateral line canal upon the head, while the majority of the others belong to a separate type of sense organs, the ampullae of Lorenzini.

Make a cut encircling the pores of the endolymphatic ducts and close to them. Do not remove this piece of skin. From it make a median incision forward to the tip of the snout and back as far as the level of the first gill slit. Starting at this incision work the skin off from the tissues beneath it. This must be a careful, close dissection. When the lateral line is reached it will be seen that the lateral line canal is continued upon the head and is joined by several others. By looking through the loosened skin toward the light the pores can be seen which lead from the canals to the surface. In the hollow of the skull in front of and above the eye is a large group of tubules which open through the pores so conspicuous at this point. At the internal end of each tubule is a slight enlargement, of denser tissue, with which a delicate nerve strand can often be seen connected. The nerve strands can be traced to a large nerve passing above the eye and distributed to the snout. These tubules are the ampullae of Lorenzini. This group of ampullae, which may be called the dorsal group, is quite definitely demarked. It will be noted that the inner ends of the ampullae are grouped in a much smaller area than their pores.

Between the spiracle and the first gill slit will be found a lateral group of similar organs. Notice the arrangement of their tubules and pores. Under the snout are two groups of ampullae on each side of the midline. The inner ventral group is separated from the outer ventral group by the lateral bar of the rostral cartilage. Some of the tubules of the outer ventral group will be found to extend to pores situated at the sides of and behind the mouth.

An adult fish possesses from 1200 to 1900 ampullae of Lorenzini. Their function is not well understood, but it has been suggested that they are organs responsive to stimuli of pressure, either of currents or water, or resulting from depth, or even of deep tones.

The system of sensory canals consists of the following members on each side of the head:—

A supra-orbital, passing above the eye to the end of the snout and bending back on the ventral surface to join the infra-orbital.

An infra-orbital, which branches off from the supra-orbital and passes ventrad between the eye and the spiracle, then turns forward along the ventral margin of the orbit, and finally bends toward the midline and extends to the tip of the snout.