240 ZOOLOGY 



SECT. 



scales. There is a distinct cloaca. The intestine contains a spiral 

 valve. The sinus venosus and the auricle are both imperfectly 

 divided into two parts. There is a contractile conus arteriosus, 

 which has a spirally twisted form, and is partly or completely 

 divided internally by a longitudinal septum. The afferent branchial 

 vessels take their origin close together immediately in front of 

 the conus. A pulmonary artery is given off from the afferent 

 branchial system; a pulmonary vein opens into the left-hand 

 division of the sinus venosus. The optic nerves form a chiasma, 

 The oviducts open anteriorly into the coelome. The ova are of 

 moderate size ; segmentation is entire. So far as is known there 

 is no metamorphosis. 



The Dipnoi are classified as follows : 



ORDER 1. MONOPNEUMONA. 



^Dipnoi in which the lung is single and the lateral jointed rays 

 of the archipterygium are well developed. 



This order comprises only the Australian Ceratodus. 



ORDER 2. DIPNEUMONA. 



Dipnoi in which the lung is double and the lateral rays of the 

 archipterygium are vestigial or absent. 



This order includes Protopterus (Fig. 863) of South Africa and 

 Lepidosiren of South America. 



3. GENERAL REMARKS. 



The three genera of living Dipnoi are closely allied in all the 

 most essential features of their structure, and it will only be neces- 

 sary now to mention the principal points in which Protopterus and 

 Lepidosiren differ from Ceratodus. Of these, Protopterus is the 

 better known, but the relationship between them is obviously so 

 close that it is unlikely that there are any important differences 

 with regard to the points that fall to be mentioned. 



The limbs (Fig. 863) 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 diverticulum of the cloaca is present and perhaps cor- 

 responds to the rectal gland of the Elasmobranchs. There are two- 

 lungs, the anterior portions of which are united to form a median 

 chamber, to which the presence, of numerous trabecula? gives a 

 spongy character. There are five branchial arches, of which the 

 last three bear the internal gills ; in addition there is a series of 

 external gills. The conus arteriosus is completely divided by a 

 longitudinal septum. The pulmonary artery is given off from 



