258 



ZOOLOGY 



SECT. 



sary now to mention the principal points in which Protopterus and 

 Lepidosiren differ from Ceratodus. 



The limbs (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 trabeculse 

 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. 914, K). In the 

 males of Lepidosiren, 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 diviaed 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 

 pores, which are very minute, lead. In Lepidosiren abdominal 

 pores are absent. 



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

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



