LARVAL GANOIDS 221 



origin of the anterior extremity of urodele ; the greatly en- 

 larged size of the opercular flap ; external gills, still promi- 

 nent ; the internal nares, OL, becoming constricted off into 

 the mouth cavity by the dermal fold of the anterior lip (as 

 in some sharks) ; and finally (as in Protopterus and some 

 batrachian larvae) the one-sided position of the anus. 



The larva of six weeks (Fig. 294) suggests the outline 

 of the mature fish ; head and sides show the various open- 

 ings of the tubules of the insunken sensory canals ; and 

 the 'archipterygium' of the pectoral fin is well defined. 

 The oldest larva figured (Fig. 295) is ten weeks old ; its 

 operculum and pectoral fin show an increased size ; the 

 tubular mucous openings, becoming finely subdivided, are 

 no longer noticeable ; and although the basal supports of 

 the remaining fins are coming to be established, there is 

 as yet little more than a trace of the ventrals. 



IV. Larval Ganoids 

 The larval forms of a Ganoid, Acipenser (Figs. 296- 

 302), resemble far more closely those of the shark than of 

 the lung-fish. When newly hatched, the young sturgeon 

 (Figs. 296, 297) is attached to the well-rounded yolk sac 

 situated in the throat region, in exactly the position one 

 would expect the yolk stalk to be situated if the yolk mass 

 were larger ; it resembles the shark larva of Fig. 295 in 

 its unpaired fin, in gill slits, in olfactory, OL, optic, OP, 

 and auditory, AU, organs, and in the fact that it possesses 

 even at this stage a trace of the neurenteric canal ; on the 

 other hand, it suggests the Ceratodus larva of Fig. 291 in 

 its stout trunk region, prominent muscle segments, pro- 

 nephros, PN, and anus, A ; at the foremost corner of the 

 yolk sac are mouth pit (stomoda;um, S) and heart. A 

 larva of the second day resembles in many features the 



