20 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



ventral position found in the elasmobranchs. On the dorsal side of the anterior 

 end of the head are two pairs of nostrils, a pair to each olfactory sac. This 

 arrangement permits a current of water to circulate through the olfactory sac. 

 The large eyes are without lids. The ears, situated behind the eyes, are invisible 

 externally. The posterior and lateral margins of the head are formed by a large 

 flap, the gill cover or operculum, which is supported by several opercular bones, 

 large, flat, scalelike bones, already noted. It covers a wide slit in the body wall 

 known as the gill opening. Attached to the ventral margin of the operculum 

 is a membrane, the branchiostegal membrane, supported by seven bony rays, the 

 branchioslegal rays. Grasp the membrane with a forceps and spread it out to 

 see the rays. Lift up the operculum and look within the cavity which it covers. 

 Four curved structures, the gill arches, which should be separated from each 

 other with a forceps, will be seen. Each bears on its outer surface a gill, con- 

 sisting of a double row of soft filaments, and on its inner margin a series of short 

 toothlike processes, the gill rakers. Thrust a probe inward between two gill 

 arches, open the mouth of the fish, and observe that the end of the probe has 

 entered the mouth cavity. The cavity of the pharynx is thus in communication 

 with the exterior through the spaces between the gill arches. These spaces are 

 gill slits corresponding to those which we saw in the dogfish and skate, but here 

 the portions of the body wall between successive gill slits have disappeared, and 

 all open into a common cavity covered by the opercuhim. This condition is 

 characteristic of all fishes except elasmobranchs. When the fish respires, the 

 mouth opens, the opercula move outward, the branchiostegal membrane unfolds 

 and closes the gill opening; water is thus drawn into the mouth and bathes 

 the gills. The mouth then closes, the opercula move inward, the branchioste- 

 gal membrane folds up, and the water passes out through the gill slits and 

 gill opening. 



3. Fins. The body is provided with median and paired fins. Of the former 

 there are an anterior and a posterior dorsal fin, a caudal fin, and a ventral or 

 anal fin. (The number and position of the median fins are very variable in differ- 

 ent fishes.) The caudal fin is apparently symmetrical with the end of the tail, 

 forming a homocercal tail. The paired fins are the same as in the dogfish. The 

 pectoral fins are located just behind the operculum, but the pelvic fins have 

 moved forward from their normal position at the level of the anus to a position 

 nearly level with the pectoral fins. Such a forward migration of the pelvic fins 

 is very common in the teleost fishes and is often associated with diminution and 

 degeneration of these fins. The anterior dorsal fin is supported by sharp hard 

 spines, and these are also present in one border of some of the other fins, but the 

 latter are supported chiefly by flexible bony fin rays. 



4. Openings. In the median ventral line just in front of the ventral fin 

 is a large opening, the anus. Behind this is a depression into which projects a 

 small elevation, the urogenital papilla. In the perch there is no cloaca, but the 



