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. Larva! 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, A U, 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 (stomodaeum, S} and heart. A 

 larva of the second day resembles in many features the 



