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 larvze) 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 Ganotds 
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, PJ, and anus, A; at the foremost corner of the 
yolk sac are mouth pit (stomodazum, S) and heart. A 
larva of the second day resembles in many features the 
