246 



TIIK SCIENCE OF LIFE. 



by means of gills. They are generally covered with 

 scales, and they have fins instead of limbs. They have 

 large immovable eyes, but no external ears. Both jaws 

 are movable. The teeth are numerous, and are gener- 

 ally recurved spines, as in the Pike ; flat and triangular, 

 with serrated edges, in the Shark ; and tessellated, in the 

 Ray. The digestive tract is relatively shorter than in 

 other Vertebrates. The blood is red, and the heart has 

 two cavities, an auricle and a ventricle, both on the 

 venous side. Ordinary fishes have four gills, the water 

 escaping by one external aperture, or "gill-slit;" but in 

 Sharks there is a separate opening for each gill. 



There are four principal varieties of fish-scales. (Fig. 

 143.) i. Cycloid scales, (cyclos, a circle,) which are most 



common ; thin, 

 flexible, and sil- 

 very, as in the Sal- 

 mon. 2. Ctenoid, 

 (kteiS) a comb,) 

 with a comb-like 

 fringe of toothed 

 processes. 3. Ga- 

 noid, {ganos, 



FIG. 143. Varieties of Fish Scales. <*. Ctenoid scale, b. Cy- brightness,) gCH- 



cioid scale, c. Ganoid scale, d. Placoid scale. erally larger than 



the preceding, and having an under layer of bone with 

 a superficial layer of enamel. Most ganoid fishes are 

 extinct. 4. Placoid, (plax, a flat plate ;) these are formed 

 of bony granules, or tubercles, or plates, the plates often 

 being furnished with spines. 



Most fishes have a series of small scales running along 



