254 



DIPNOI 



provided with a considerable amount of yolk,- arid surrounded by a 

 gelatinous coat secreted by the oviduct, undergo total but unequal 

 cleavage. The larvae, except in Ceratodus, have feathery external 

 gills and a ventral sucker of attachment below the head (Fig. 216). 



The structure of the Dipnoi may be summarised as follows : 

 As primitive characters : the spiral valve, the contractile conns, the 

 cloaca, the unconstricted notochord, the ventral nostrils. As specialised 



Fio. 224. 



Diagmnis illustrating the formation of the vena cava in the Dipnoi and Amphibia (after 

 Kellicott). A, Ceratodus, early larva; B, Cerutodus, later stage; C, Ceratodus, adult; D, 

 1'i'otoptcrus; E, Salamaiulra; F, liana, a.c, anterior cardinal ; rf.c, ductus Cuvieri ; h, hepatic 

 veins ; l.p.c, left, and r.p.c, right posterior cardinal ; v.c, vena cava. 



characters : the camptotrichia, the loss of the spiracle and pineal eyes, 

 the reduction of the hyomandibular, the autostylism, the loss of the 

 marginal jaw-bones and teeth, the permanent grinding plates formed 

 from coalesced internal teeth, the median pelvic cartilage, the twist- 

 ing of the air-bladder to a dorsal position, the special pulmonary 

 circulation, the incipient vena cava inferior, the large paired cerebral 

 hemispheres. It must be mentioned also that the scales are rounded, 

 the notochordal sheath is invaded, the median fm-radials articulate 

 with the axial skeleton, pleura! ribs only are present, the paired 

 fins have a jointed median axis, and the air-bladder has a ventral 

 opening. 



