THE SMELT 305 



In addition to the axial skeleton there is the skeleton of the 

 anterior and posterior appendages. The skeleton of each 

 pair of appendages is borne upon a bony ring which nearly 

 encircles the body. These are called respectively the shoulder 

 and pelvic girdles. The skeleton of the fin consists of a row 

 of cylindrical bones which bear at their outer extremities 

 the horny rays that support the fins. 



Organs and function of nutrition (Fig. 287). — The large 

 mouth bears teeth not only on its jaws but also on its roof {vo). 

 The whole of the interior of the mouth was, in the ancestors 

 of fishes, covered with scales of the same sort as those that 

 covered the body just outside the mouth. All teeth have their 

 origin in such fish-scales. In early fishes the number of teeth 

 was large and the position undefined, but in the higher animals 

 the number has become limited to a single row on each jaw. 

 At the hinder part of the mouth are four vertical gill-slits 

 (ph), fringed by a bony straining apparatus, the g-iU-rakers. 

 The water that has been taken in through the mouth is forced 

 out through the gills by closing the mouth in front and raising 

 its roof so as to exjDcl the water. The remainder of the food- 

 canal consists of a stomach that is twisted on itself and of 

 an intestine which usually forms either no convolutions or 

 only the simplest. The liver Qr) communicates by a tube 

 with the anterior part of the intestine, and is provided with 

 a large gall-bladder {g.hl). Immediately below the liver 

 opening is a bunch of hollow tubes called the caeca (py.c). 

 These are also digestive in function. An air-bladder (aM) Hes 

 just over the intestine, and in front it opens into the pharynx 

 (pn.d). The air-bladder of fishes functions so as to enable 

 the fish to vary the volume of gases in the body and thus to 

 vary its specific gravity. It is often spoken of as an organ of 



