CLASS PISCES 



351 



371. Chondrostei. — In the Chondrostei, or cartilaginous ganoids, 

 the pectoral fins are not used to support the body, and the tadpole-like 

 larva has been considerably modified. The skeleton is largely car- 



FiG. 245. — A crossopterygian. Polypterus senegalus Cuvier. o, the adult. X H- 

 h, the larva. X 2%. {a from Bridge, "' Cambridge Natural History," b after Budgett, by 

 the courtesy of The Macmillan Company.) The latter figure does not show the fact that 

 the gills are alternately long and short. The arrow and line in Fig. a point to the position 

 of the left spiracle. 



tilaginous, but the cartilage is overlaid with dermal bones. The sturgeons 

 (Fig. 246) and spoonbills are the living representatives of this group. 



Fig. 246. — Lake sturgeon, Acipenser rMWcMW^MSiLeSueur. X Hs- {From Jordan, "Guide 

 to the Study of Fishes," by the courtesy of D. Appleton & Company.) 



372. Holostei. — The Holostei, or bony ganoids, have a completely 

 ossified skeleton. In some the scales form a complete armor. Examples 



Fig. 247. — Alligator gar, Lepisosteus tristoechus (Block and Schneider). X J^eO- (From 

 Jordan, "Guide to the Study of Fishes," by the courtesy of D. Appleton & Company.) 



are the bowfins, and the gars (Fig. 247). The development of the bow- 

 fin betrays primitive characteristics. The eggs are nearly holoblastic, 

 and the early development is rather more like that of a lobe-finned 



