148 GASTROPODA. 



is found in the Pteropoda that have a sinistrally twisted shell, but 

 in other respects show the structure of dextrally twisted forms ; 

 these have the operculum also sinistrally twisted, whereas spiral 

 opercula elsewhere always have a twist opposite to that of the shell 

 (Pelseneer, No. 86). 



[Forms with a dextral organisation in a sinistral shell, and which 

 are supposed to have arisen as above, have been termed ultra-dextral. 

 The commencement of an ultra-sinistral coiling is seen in Planorbis 

 context, which possesses a true sinistral organisation with a shell 

 which otherwise would be regarded as a flattened dextral coil. The 

 embryo of this Gastropod, however, possesses a well-marked sinistral 

 shell.] 



The asymmetry characteristic of the Gastropoda may, however, be- 

 come more or less marked by the acquisition of a secondary bilateral 

 symmetry. This is the case in forms which, like the Pteropoda, have 

 become adapted to a free-swimming manner of life. In such cases 

 the principal cause of the asymmetry, which we found to be the 

 creeping manner of life in connection with the development of a 

 high visceral mass, falls into the background. The fact, however, 

 that there are Gastropods which again become almost symmetrical 

 while still leading a creeping life, but in which the shell has al- 

 together or partly degenerated, as is the case in Onrhidium and the 

 Limaeidae, shows what an important part is played in these processes 

 by the covering of the body. 



5. The Development of the External Form of the Body in 

 the Different Divisions of the Gastropoda. 



A. Prosobranchia. 



We have already repeatedly alluded to the development of the 

 larval form of the Prosobranchia and to its transformation into the 

 adult,* the principal features in these processes, the development of 

 the Troclwphore and Veliger larvae and their transformation into the 

 adult are thus known to the reader. Certain divergences, however, 

 occur among the Prosobranchia, especially in the earlier ontogenetic 

 stages, causing a modification of the external form of the body, and 

 thus requiring special consideration. 



It has already been stated (pp. 112, 116) that the eggs of many 

 Gastropods are very rich in yolk, and this influences not only the 



* @f- PP- 123, 131 and 134 on the development of Patella, Vermel us and 

 I'aliulina, also Figs. 49-59. 



