U'Nd-FISH 



377 



inu (Ceratodus), or when on the otlu-r hand tin- waters have completely 

 -dried up and left the lung-fishes ensconced in their burrows in the dry 

 mud (Lepidosiren, Protopterus) it is able to meet 

 the entire respiratory requirements. 



The digestive part of the alimentary canal is 

 comparatively short and as in Elasmobranchs 

 and Crossopterygians there is a spiral valve. In 

 the young larva (Fig. 162) the endodermal rudi- 

 ment of the intestine is coiled in a corkscrew 



spiral. The turns of this later become fused li 



together, a last vestige of the spiral twisting 

 remaining as the spiral valve in the interior. 

 This mode of development suggests the probable 

 evolutionary meaning of the spiral valve occur- 

 ring in so many archaic vertebrates, namely that 

 it is a last reminiscence of a common ancestor of 

 existing vertebrates in which the intestine was 

 relatively long and spirally coiled. 



The kidney of the Lung-fish during larval 

 stages is a pronephros, usually with two tubules 

 on each side when at the height of its develop- 

 ment. In the adult this is replaced by a long 

 narrow opisthonephros, the ducts of which fuse 

 together at their hind ends to form a dilated 

 bladder or caecum opening into the cloaca on its 

 dorsal side. 



The male reproductive organs are of special 

 interest. The testes are somewhat cylindrical 

 bodies which stretch through the greater part of 

 the length of the peritoneal cavity. Towards 

 their posterior ends they narrow suddenly into a 

 sterile portion and in Lepidosiren there pass off 

 from this a series of about half a dozen vasa 

 efferentia which communicate with certain of the 

 tubules close to the hind end of the kidney. In 

 Protopterus the general arrangement is similar but 

 here the vasa efferentia are reduced to the last one of the series on each 

 side. In the latter animal the spermatozoa pass from the anterior func- 

 tional part of the testis back in turn through the slender sterile portion, 

 the vas efferens ; the kidney tubule, to the kidney duct and so to the cloaca. 



FIG. 162. 



Dissection of a larva 

 of Lepidosiren showing the 

 spiral coiling of the enteric 

 rudiment, g.b, Gall-bladder ; 

 li, liver ; V, ventricle. 



