138 



THE ANATOMY OF VERTEBEATED ANIMALS. 



them, as in the Elasmobranchii ; but their extremities project 

 freely beyond the edge of the septum which separates each 

 pair of branchial clefts, as in the Teleostei ; and, as in the 

 Teleostei, they are co\-ered by a bony operculum, 



h. There is a large air-bladder connected by a perma- 

 nently open pnewmatic duct with the oesophagus, as in many 

 Teleostei. 



c. As in the Teleostei, there is no cloaca. 



The ventral fins are always abdominal in position. The 

 tail is diphycercal, or heterocercal, and the terminal portion 

 of the notochord is not ossified. The cavity of the abdomen 



Fig. 38. 



Fig. 38. — The brain of Lepidostews semiradiatiis. A. From above ; B. 

 From below :— /, the medulla oblongata ; rf, the cerebellum ; c, the 

 optic lobes of the mesencephalon ; g, the cerebral hemispheres ; 

 li, the pituitarj' body ; /, the lobi inferiores. Ch, the chiasma; I., ol- 

 factory ; II., optic nerves. 



is placed in communication with the exterior by abdominal 

 pores. Finally, the dticts of the reproductive organs com- 

 municate with those of the permanent urinary apparatus, 

 which is, in part, an Elasmobranch, in part, an Amphibian, 

 character. 



The exoskeleton presents the most extreme variations in 

 the Ganoidei. Spatularia is naked ; Accipenser and Scapi- 

 rhynchus develope numerous dermal plates composed of true 

 bone ; Amia is covered with overlapping cycloid scales ; 

 Lepidosteus and Polypterus have solid, rhomboidal, enamelled 



