250 ZOOLOGY SECT- 



overlapping cycloid scales. There is a distinct cloaca. The 

 intestine contains a spiral valve. The auricle and the sinus venosus 

 are each 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 imme- 

 diately in front of the conus. A pulmonary artery is given off 

 from the afferent branchial system on either side ; a pulmonary 

 vein opens into the left division of the auricle. The optic nerves 

 form a chiasma. The oviducts open anteriorly into the coelome. 

 The ova are of moderate size ; segmentation is entire. 

 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. 924) 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 

 necessary now to mention the principal points in which Protopterus 

 and Lepidosiren differ from Ceratodus. 



The limbs (Fig. 924) 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, derived developmentally 

 from the urinogenital sinus, is present, and perhaps corresponds 

 to the sperm-sacs 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 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 Protopterus (Fig. 925, K). In the males of Lepidosiren, 

 vascular filaments, which may be accessory respiratory organs, are 

 developed on the paired fins during the breeding season. The conus 

 arteriosus is completely divided by a longitudinal septum. The 

 pulmonary artery is given off from the point of union of the 



