172 



GASTROPODA. 



velum and the foot, and the posterior ring immediately in front of the 

 aperture of the shell. In this case, the posterior part of the body is 

 still of some length ; in other larvae, the posterior ciliated ring is 

 found almost at the end of the body (Fig. 76 B). The velum seems to 

 bear no relation to the ciliated rings. After it degenerates, the larva 

 presents an appearance which, for a Mollusc, is very peculiar, recalling 

 rather the Annelid larvae which are encircled with several ciliated 

 rings. These also are at a stage following the Troc/tvphure larva, 

 as already mentioned (Vol. i., p. 277), and as we were able to see 

 in various polytrochan larvae. This comparison to an Annelid 

 larva has already been instituted by GEGENBAUB, and the fact has 

 been emphasised that the resemblance is accidental and of no great 

 significance. 



Our knowledge of the ontogeny of the Gymnosomata relates entirely to CUow 

 and Pneumodermon, two forms which seem to agree pretty closely in the 

 general features of their development, as shown by JOH. MULLER, GEGENBAUK, 

 KBOHN and FOL. As most of these larval Gymnosomata have not been traced to 

 the adult stage, it is by no means certain that the larvae examined belonged 

 to these genera. 



The mouth lies on the anterior, proboscis-like projection, and the 

 anus, which is displaced to the right, ventrally between the first and 

 second ciliated rings. Two pointed outgrowths lying near the mouth 

 represent the rudiment of the so-called cephalic cone (Fig. 77 Z^). 



Somewhat further back, but 

 in any case in front of the 

 anterior ciliated ring, the rudi- 

 ments of the acetabuliferous 

 appendages appear (Jon. 

 MULLER). [These, according 

 to PELSENEER, are derivatives 

 of the proboscis.] When the 

 proboscis is evaginated at a 

 later stage, these seem shifted 

 further back, being now situ- 

 ated on its posterior part 

 (Fig. 77 B). The foot was 

 previously referred to as a pointed ventral appendage, lying behind 

 the first ciliated ring. Before this stage, an anterior, horseshoe- 

 shaped lobe forms in the posterior concavity of the pointed part 

 of the foot. Immediately behind the anterior lobe of the foot, on 

 either side of the posterior lobe, the first rudiment of the fins can 



B 



FIG. 77. Two larvae of Pneumodermon at 

 different ages (after GEGENBAUR, from 

 BALFOUK'S Text-book), an, anus. 



