140 



THE GASTROPODA 



Euthyneura and in the majority of Streptoneura. Even in those 

 Gastropods that are hatched out in the adult form, the veliger stage 

 can generally be recognised, in a more or less reduced condition, 

 within the egg membranes, e.g. in Buccinum, Cenia, the basommato- 

 phorous Pulmonates. In all Gastropods the velum is reduced in 

 proportion as the foot develops; nevertheless, in a considerable 

 number of pelagic larvae the veliger stage is preserved for a long 

 time, and the velum persists, and often develops excessively long 

 lobes even after the creeping foot is fully and normally developed : 

 such is the case in " Macgillivraya " (Fig. 121), " Agadina" " Chele- 

 tropis," "Sinusigera" " Echinospira" etc., all of which are special 

 pelagic larval forms of Streptoneura which were long considered to 



FIG. 121. 



" Macgillivraya," pelagic larva of a siplionate 

 Streptoneurous Gastropod (Dolium), ventral aspect, 

 X 12. /, foot ; m, mouth ; sh, shell ; si, siphon ; te, 

 tentacle and eye ; re, Jobes of the velum. (After 

 MacDonald.) 



Fio. 122. 



Shell of a young 

 Purpura haemastoma, 

 enlarged, dorsal aspect, 

 ca, canal of the adult ; 

 e.s, limit of the embry- 

 onic shell ; sp, spire. 

 (After Dautzenberg.) 



be distinct genera. The velar lobes may even produce lobate 

 expansions of the margin of the aperture of the shell, but these dis- 

 appear when the velum is absorbed and the shell assumes the adult 

 form (Fig. 122). 



IV. DEFINITION. 



The asymmetry of some of the principal organs of the body is 

 the chief characteristic of the Gastropoda. The essential feature of 

 this asymmetry is that the anus generally lies to one side of the 

 median plane ; that the ctenidium, the osphradium, the hypo- 

 branchial gland, and the auricle of the heart are azygos, or at least 

 are more developed on one side of the body than the other ; and 

 that there is only one genital orifice, which lies on the same side of 

 the body as the anus. In other words, one-half generally the 

 morphologically left but topographically right half of the anal 

 complex is either atrophied or has disappeared altogether. This 

 asymmetry, expressed by the transfer of the morphologically right 



