38 ELEMENTS OF COMPARATIVE ZOOLOGY. 



SUBCLASS III. GADOIDS. 



These are remnants of a group once very abundant on 

 the world's surface, but now showing less than fifty living 

 species in the whole world, and most of these in North 

 America. Some of them are much like Selachians, others 

 like Teleosts, and still others go off towards the Holocephali. 

 The skeleton is bony or cartilaginous; the body may be 

 covered with ganoid or cycloid scales, or with bony plates, 

 or it may be naked; the tail either homo- or heterocercal ; 

 the gills are covered with an operculum. The heart is 

 provided with an arterial cone, and the intestine has a spiral 

 valve. A swim bladder occurs, and this has its duct, which, 

 in one form, empties into the ventral side of the O3sophagus. 

 With this confusing mixture of characters it is not strange 

 that many naturalists have split up the group and distrib- 

 uted its members among the other subclasses. 



FIG. 16. Common Sturgeon (Acipenser sturio). After Goode. 



To it belong the sturgeons, the most sharklike of all, 

 some of which live in fresh water, while the marine forms 

 ascend the rivers to lay their eggs. From their ovaries are 

 made caviare, while their swim -bladders furnish the isinglass, 

 now so largely supplanted in domestic economy by gelatine. 

 Though some attain an enormous size, all feed upon small 

 animals, worms, insect larvae, etc=, which they find in the 

 mud. The garpikes, with their strongly armored bodies, 

 which also belong here, on the other hand, are very vora- 



