258 



Zoor.OGY 



SECT. 



sary now to mention the principal points in which Protopterus and 

 Lepidosiren differ from Ceratodus, 



The Hmbs (Fig. 913) are long and very narrow, and the limb- 

 skeleton is correspondingly modified, consisting of a slender, jointed 



axis without, or with only 

 vestiges of, the lateral rows of 

 rays. A blind dorsal diverti- 

 culum of the cloaca, derived 

 developmentally from the 

 urinogenital sinus, is present, 

 and perhaps corresponds to 

 the sperm-sacs of the Elasmo- 

 branchs. There are two lungs, 

 the anterior portions of which 

 are united to form a median 

 chamber, to which the pre- 

 sence of numerous trabecidse 

 gives a spongy character. 

 There are five (or six) reduced 

 rod-like branchial arches, of 

 which the last three bear the 

 internal gills ; in addition 

 there is a series of external 

 gills in the larva, vestiges of 

 which persist in the adult Pro- 

 topterus (Fig. !)14,/ir). In the 

 males of Lei^idosiren, vascular 

 filaments, which may be ac- 

 cessory respiratory organs, are 

 developed on the paired fins 

 during the breeding season. 

 The conus arteriosus is com- 

 pletely divided by a longi- 

 tudinal septum. The pulmon- 

 ary artery is given off from 

 the point of union of the 

 efferent branchial arteries 

 into a single lateral trunk. 

 In Protopterus there is usually 

 a single abdominal pore open- 

 ing on the dorsal wall of the 

 cloaca ; this leads into a cavity 

 into which the true abdominal 

 In Lepidosiren abdominal 



« 



+> 



u 



« 



a 



0) 



o 



o 

 u 



minute, lead 



pores, which are very 

 pores are absent. 



The brain in both Lepidosiren and Protopterus, as well as in 

 Ceratodus, differs from that of Fishes in general, and resembles 



