174 PISCES. 



a membranous border supported by rays, or (Ceratodus), like the fins 

 of the Crossopterygians, consist of a central snaft covered by scaly 

 skin, and of a border provided with rays. The pelvic fins are placed 

 far back. In front of the anterior pair of fins there is a gill slit on 

 either side, above which in the African genus Protopterus (Rhino- 

 cryptis) three external gill tufts are retained till late in life. In the 

 Brazilian genus Lepidosiren external gills are absent. 



The Dipnoi show themselves to be fishes by the possession of gills 

 as well as by the external form. There are either four gills (Cera- 

 todus) as in fishes, or their number is reduced. The structure of 

 the skeleton points decidedly to the Ganoids, to which the Dipnoi 

 are in other respects closely related. In Lepidosiren the notochord 

 persists as a continuous cartilaginous cord, from the fibrous sheath 

 of which dorsal and ventral bony arches with ribs project. In front 

 the notochord is prolonged into the base of the skull, which remains 

 at the stage of the primitive cartilaginous cranium. It is, however, 

 covered by some osseous pieces. The facial bones of the head are 

 much more developed, especially the jaws, whose teeth consist, as in 

 Ckimcera, of perpendicularly-placed cutting plates or (Ceratodus) 

 recall those of Cestracion. The intestine contains a spiral valve, 

 which terminates at some distance from the cloaca. The cloaca 

 contains the sexual opening, and the openings of the ureters in its 

 side walls : it opens to the exterior sometimes to the right side, 

 and sometimes to the left, and on its posterior side there is in Lepi- 

 dosiren an independent urinary bladder. 



On the other hand, the respiration by means of lungs and the 

 structure of the heart indicate a relationship to the naked Amphibia. 

 The cartilaginous nasal capsules, as in all lung -breathing animals, 

 open behind into the mouth by apertures, which perforate the roof 

 of the mouth, and are placed far forward, directly behind the extre- 

 mity of the snout. The swimming-bladder is represented by two 

 sacs (in Ceratodus only one) placed outside the body cavity, ventral 

 to the kidney, and opening into the ventral wall of the pharynx by 

 means of a short common duct. These sacs function as lungs, 

 inasmuch as they obtain venous blood from a branch of the posterior 

 aortic arch and return arterial blood to the heart by pulmonary 

 veins. To this agreement with the Amphibia may be added the 

 similar arrangement of the heart and the principal trunks of the 

 vascular system, the incompletely divided right and left auricles 

 and the double circulation. There is a muscular conus arteriosus 

 which either has an arrangement of valves like that in the Ganoids 



