THE FISHES 159 
largely in burrowing. Like the lancelet and other simple 
chordates, they are of the greatest interest to the zoologist 
who gains from them some idea of the lowly vertebrate 
forms that peopled the earth long ago. 
151. True fishes——The third class, Pisces or true fishes, 
to which the shark as well as the bass and perch belong has 
a well-developed skeleton, skull, and brain. The lower jaw 
is developed, forming a distinct mouth, and there is at least 
a shoulder-girdle and pelvis; although the fins these should 
bear are not always developed, the general traits are those 
we associate with the fish. Of the true fishes, there are 
again several strongly marked groups, usually called sub- 
classes. Of these, three chiefly interest us. 
152. The sharks and skates——Very early in the life of 
the sharks (Fig. 100) and skates (Selachii or Elasmobranchit) 
Fie. 100.—Dogfish (Squalus acanthias). One-seventh natural size. 
a notochord appears, similar to that in the lancelet and the 
lampreys. As growth proceeds its sheath becomes broken 
up into a series of cartilaginous rings, which thus appear 
like spools strung on‘a cord. As the fish grows older these 
“ spools” or vertebre grow solid, cutting the notochord into 
little disks, and great flexibility is thus secured. Cartilagi- 
nous appendages also grow up and cover the spinal nerve- 
cord lying above, and give strength to the unpaired fins; 
the paired fins also have their supports. The shoulder- 
