128 VERTEBRATE ZOOLOGY 



5. The vertebral column is not very compact, the vertebrae 

 being often without a centrum; if the latter is present it is an arch- 

 centrum. 



6. The fins are supported by bony dermal rays. 



7. The integument is characterized by scales without the denticle 

 or spike seen in the placoid type of scale. They are ganoid, cyloid, 

 or ctenoid in form and may be either tessellated (laid Uke tiles) or 

 imbricated (overlapping like shingles), the former being the more 

 primitive condition. 



8. No claspers are known in the group, though in some groups 

 the pelvic fin may be modified as an intromittent organ used in sexual 

 copulation. 



9. Most members of the sub-class have an air-bladder, which 

 serves to compensate for the additional weight caused by the ossified 

 skeleton. 



10. The gill filaments project beyond the edges of the inter- 

 branchial septa. 



11. The nasal sacs have no naso-oral grooves and they open by 

 separate nares. 



12. The brain has a much reduced cerebrum, with small oKactory 

 lobes. 



13. No cloaca. 



14. The ova are usually small and numerous and range, with 

 respect to the cleavage, from holoblastic to meroblastic types, the 

 more primitive types resembhng the eggs of Amphioxus in that they 

 have holoblastic cleavage. 



Practically all of these characters show an evolution away from the 

 elasmobranch condition. In some cases the evolution is progressive 

 and in others regressive. 



Oedek I. Crossopteeygii (Lobe-Finned Ganoids) 



This very important group which was abundant in Devonian times 

 and is represented to-day by Polypterus and Calamychthys, is the 

 most primitive division of the Teleostomi, and, from the phylogenetic 

 standpoint, much more significant than either of the other ganoid 

 orders or the teleosts. There are evidences that this order of fishes is 

 the ancestral group not only of all the higher fishes, but of the terres- 

 trial vertebrates. 



