210 BRANCH CHORDATA 



(see Fig. 170, Rem'ora), the scales are either ctenoid or cycloid. 

 These fishes vary in shape. They \'ary in size from our little 

 darter, I5 inches in length, to the "horse-mackerel," which may 

 weigh as much as a cow. They differ in habits from the pre- 

 daceous, swift pikes and pickerels to the peculiar flounder on 

 the bottom of the sea. 



The Remora (Fig. 170) is a lazy fish. It has a sucker on 

 top of its head, by which it holds fast to sharks or larger fishes, 

 and thus saves itself the effort of locomotion. 



Order I. Crossopteryg'ii. — There are only two existing genera, 

 Polyp'terus and Calamoichthys, of Africa. 



Order II. Chondros'tei (Sturgeons) (Fig. 171). — They have 

 paired fins with no basal lobe, supported by dermal rays. The 

 pelvic fins are abdominal. The vertebral column consists of the 

 notochord with cartilaginous arches. The tail is heterocercal. 



Fig. 171. — Common sturgeon {Aci pcH'xcr slu'rio). (Report U. S. P. C, 



1S'.)9.) 



The mouth is ventral, projectile, and toothless, and sucks up 

 worms and larvse from the muddy bottom. The surface is 

 roughened by separate scales and by five rows of bom- plates. 



Sturgeons are found in streams and lakes of the Northern 

 Hemisphere and are the largest fresh- water fishes. Those of the 

 lower Columbia River sometimes weigh from 800 to a 1000 

 pounds. 



From the swim-bladder of the sturgeon, glue, cement, court- 

 plaster, and isinglass are made. The egg-masses, called roe, 

 furnish caviare. 



Order III. Holos'tei. — Familiar examples of this order are the 

 gar-pike and the mud-fish, often called dog-fish, of the streams 

 of the central states. 



The skull is ossified. The scalers are ganoid or cycloid; the 

 tail, diphy cereal or homocercal. The pelvic fins are abdominal. 



