248 CLASSIFICATION OF VERTEBRATES. 



of the ccelom. In the males there is either a simple tube which 

 connects each testis with .the urogenital sinus, or there may 

 intervene between tube and testis a system of smaller canals, 

 the vasa efferentia. 



Legion I. Ganoidea. 



Teleostomes in which the body is either naked or covered 

 with ganoid or cycloid scales, or bears bony plates ; the skeleton 

 either largely cartilaginous or well ossified ; the tail diphy-, hetero- 

 or homocercal ; ventral fins always abdominal in position ; fulcra 

 present in most recent and in fossil forms ; swim-bladder with 

 duct ; intestine with a spiral valve ; an optic chiasma present ; 

 heart with a conus arteriosus ; eggs with a total segmentation. 



The group of ganoids contains but a few recent forms, the 

 remnants of a much larger fauna in past times. Its members 

 are widely distributed over the globe, North America, however, 

 having the greater proportion of the species, most of which are 

 inhabitants of fresh water. In the definition above fulcra are 

 mentioned. These are spine-like scales upon the anterior sur- 

 faces of the fins. 



So far as they have been studied, the eggs of the ganoids 

 undergo a total segmentation ; but, owing to the presence of a 



FIG. 248. Segmenting egg FIG. 249. Larva of Amia, about the time of 



of Amia, showing the unequal h-itching, showing the sucking disk at the tip of 

 cleavage, after Dean. the snout, after Dean. 



large amount of yolk, the resulting cells are very unequal in 

 size, the large cells being at one pole of the egg, the smaller at 

 the other. In the sturgeon the central nervous system develops 

 as a tube ; but in the other forms it is at first a solid keel, and 

 only later does a lumen appear by splitting. The larvae of 



