VERTEBKATA. 
313 
may be recurved spines, as in the Pike; flat and triangu¬ 
lar, with serrated edges, in the Shark; or flat and tessel¬ 
lated in the Ray. They feed principally on animal mat¬ 
ter. The digestive tract is relatively shorter than in other 
Vertebrates. 160 The blood is red, and the heart has rarely 
more than two cavities, an auricle and a ventricle, both on 
the venous side. Ordinary Pishes have four gills, which 
are covered by the operculum , and the water escapes from 
an opening behind this. In Sharks there is no operculum , 
Fig. 285.— Salmou (Salmo salar). Both hemispheres. 
and each gill opens separately. The brain consists of sev¬ 
eral ganglia placed one behind the other, and occupies but 
a small part of the cranial cavity. Its average weight to 
the rest of the body may be as low as 1 to 3000. The 
eggs of bony Fishes are naked and multitudinous, some¬ 
times numbering millions in a single spawn; those of the 
Sharks are few, and protected by a horny shell (Fig. 164). 
There are about thirteen thousand species of Fishes, of 
which over two thirds are Teleostei. There are two sub¬ 
classes of Pisces. 
