The spindle-shaped body tapers from near the middle toward both head and tail; the head is flattened on both the dorsal and ventral sides, while the remainder of the body is nearly round, with a lateral compression which is not pronounced except in the caudal portion.
The general color of the back and sides is gray; darkest above, where the skin is spotted with scattered, small, round, light spots. The color of the upper parts shades into the yellow white of the ventral surface.
Can head, trunk, and tail regions be distinguished? If so, what characters determine the extent of each?
A little above the middle of the side of the body is the lateral line, (distinguished partly by color, partly by being slightly elevated), which extends from the back of the head to the tail. Cut through the skin across the lateral line at several points along the body and notice the canal which lies in the dermis under the lateral line. This is the lateral line canal, which opens to the surface by numerous pores (too minute to be seen), and contains a series of special sense organs along its dorsal and inner surfaces. Near the base of the caudal fin the lateral line canal passes into a groove which continues the lateral line to within a short distance of the edge of the fin. The lateral line canal in its development begins as a groove along the side of the body which becomes closed by the fusion of its edge except in this terminal portion.
The open groove does not appear in Eugaleus.
In the midline of the body are two triangular dorsal fins, each attached to the body for about half its length. The basal portion of each is thick and muscular, and contains supporting cartilages embedded in the muscles. The remainder of the fin is flexible and semi-transparent, horny fin-rays being faintly visible between the layers of skin. In front of each fin is a strong spine which seems to serve both as a cutwater and a weapon of defense. (Eugaleus has no spines.)
The broad, paired, pectoral fins, having the general characteristics of the dorsals, spring from the ventral edges of the body just back of the head. A hard bar of cartilage connecting the bases of the pectoral fins can be felt through the skin of the ventral surface of the body. This is the ventral part of the pectoral girdle.
Farther back, and also on the ventral surface, are the paired pelvic fins. The pelvic girdle can be felt through the skin between the bases of these fins. If the specimen is a male, it will have a fingerlike process projecting backward from the base and along the inner side of each pelvic fin. These organs, which attain a considerable size in adults, are modified portions of the pelvic fin used as copulatory organs. They are named variously claspers, myxopterygia, or pterygopodia. A groove runs along the dorsal side of the clasper from the tip to near the base, where it opens into a long sac (glandula pterygopodia) extending some distance in front of the pelvic fins just within the skin of the ventral surface. By feeling a cartilaginous axis of the clasper can be discovered, which extends to the tip of the organ. On the dorsal surface and near the end of the clasper is a sharp grooved spine on the outer side, and a strongly recurved hook on the inner side, both almost hidden by a flap projecting from the inner edge of the groove. This flap is stiffened by a series of small cartilages similar to the radial cartilages in the bases of the fins.
The caudal fin is asymmetrical, extending along the dorsal and ventral edges of the posterior end of the body. Observe the upward bend of the vertebral column which occurs in the tail; it is this character which marks the heterocercal type of tail.
Eugaleus has a median ventral, or anal, fin a short distance anterior to the caudal fin.
The mouth is a broad transverse slit upon the ventral surface of the head. The cartilaginous jaws can be seen and felt just within the mouth. Both upper and lower jaws are armed with rows of flat, sharply pointed teeth. Study their arrangement and approximate number. Jaws of previously dissected specimens should be examined under a dissection microscope. The exact form and arrangement of the functional teeth can then be ascertained easily, and an examination of the inner surface of either jaw will disclose several rows of developing teeth. As the young teeth develop they move, a row at a time, into position on the edge of the jaw; the oldest teeth, occupying the outer row, are shed at about the same time.
The upper jaw is partly overhung by a lip-like fold of skin. At each side of the mouth is a pocket, directed obliquely, having no communication with the mouth. These labial pockets provide places for the labial cartilages (which can be felt along the medial edges) when the mouth is closed, and also afford freedom of motion to the mandible. Cut along the inner edge of the labial pocket and expose the cartilages for examination. (The labial pocket of Eugaleus is much smaller, and in front of the corner of the mouth rather than lateral to it. The two cartilages are completely separated from each other, the posterior one scarcely reaching the pocket.)
In front of the mouth are the nostrils, their apertures apparently divided by projecting flaps of the anterior margin. Explore the cavity of the nostril with a probe to get a good idea of its size and form.
Between the pelvic fins is the cloaca, a large depression into which open the alimentary canal, the excretory and genital ducts, and the abdominal pores. The opening of the alimentary canal, the anus, is at the anterior end of the cloaca. In preserved specimens part of the intestine is frequently everted through the anus. A large fleshy process, bearing a pore at its tip, projects from the dorsal wall of the cloaca. In the male this is the uro-genital papilla; in the female the urinary papilla. In the female a genital pore, the opening of the oviduct, is found on either side of the papilla. An abdominal pore, leading into the abdominal cavity, is found on each side of the cloaca at the posterior margin. These are frequently closed in young specimens.
The cloaca of Eugaleus has a comparatively small opening upon the ventral surface, which must be enlarged before the parts described can be seen well.
The laterally placed eyes are without lids; observe the considerable difference in the amount of curvature of the dorsal and ventral margins of the eye.
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.
A hyomandibular, which leaves the infra-orbital below the eye and runs back beyond the angle of the mouth.
A short mandibular, on the mandible close to the angle of the mouth, which is not connected with the other canals.
The canal systems of the right and left sides are connected by a supratemporal canal just behind the endolymphatic pores, and often by an anastomosis of the infra-orbital canals in front of the mouth.