DIPNOI. 271 



In the venous system the most marked advance is the presence 

 of a postcava, while but a single (left) postcardinal comes to 

 complete development. A renal portal system is present. 



The mesonephros is elongate in Protopterus, short in Cerato- 

 dus ; and nephrostomes are lacking in the adult. Its duct is 

 thick walled, and is apparently a Wolffian duct, although em- 

 bryological evidence is as yet lacking. The gonads are elon- 

 gate, and attached to the lateral parts of the mesonephros. The 

 oviducts are elongate and contorted, and open into the coelom 

 far forward by narrow ostia. Posteriorly they unite just in 

 front of the cloaca. In Ceratodus no vasa deferentia occur, the 

 spermatozoa apparently passing out through the pori abdomi- 

 nales. In Protopterus a well-developed duct occurs in connec- 

 tion with either testis, each passing behind into the rudimentary 

 Miillerian duct, and thence by a common trunk into the cloaca. 

 The cloaca also bears an azygos diverticulum (Fig. 40), usually 

 regarded as an urinary bladder (cf. however, the rectal gland of 

 elasmobranchs). 



Of the development of Protopterus nothing is known. The 

 segmentation and external development of Ceratodus have been 

 studied, and show striking similarities to that of Petromyzon, 

 and especially to the amphibia. The egg undergoes a total but 

 unequal segmentation, while gastrulation is effected by over- 

 growth as in the amphibia, the result being, as in that group — 

 the formation of an elongate primitive groove, on either side of 

 which the medullary folds arise. These close in, and gradually 

 the embryo arises as a ridge above the yolk. So far as is known 

 no metamorphosis occurs. 



ORDER I. ARTHRODIRA. 



Body in front covered with large bony plates, a dorsal pair 

 articulating by a hinge joint with the cranium ; paired fins rudi- 

 mentary or absent ; pelvis represented by a pair of club-shaped 

 plates. 



The relationship of the arthrodira to the other dipnoi is not 

 beyond question. The group is restricted to the palaeozoic 

 rocks, and remains are abundant in Europe and in America. 



