DEVELOPMENT OF THE EXTERNAL POEM — PULMONATA. 1 75 



The operculum, on the contrary, is wanting according to Joyeux- 

 Laffuie (No. 51) and the foot which, even in the Veliger stage, is 

 very large is also covered at its anterior and dorsal side with delicate 

 cilia. The shell is thrown off during embryonic life, and the velum 

 also degenerates within the egg-shell. 



With regard to the absence of the operculum which, according to 

 Joveux-Laffuie can hardly be doubted, it should he pointed out 

 that this organ is as a rule not found in the I'ulmonates. 



The marine Arnphibola, however, lias an operculum showing the usual 

 structure and position (i.e., lying posteriorly on the back of the foot, No. 66). 

 Unfortunately, this Australian form is little known ; a more accurate know- 

 ledge of its anatomy and ontogeny is very desirable. According to Semper 

 (No. lis, ii.. p. 100), the embryos of Auricula and Scarabus have opercula. 



In Onchidium, after the shell has been thrown off, the mantle, 

 with the reduced pulmonary cavity, shifts dorsally and, with the 

 kidney, opens by a median aperture at the posterior end of the body. 

 The hitherto asymmetrical aims (lying on the right side) also assumes 

 a median position at the posterior end of the body. In some species, 

 the pulmonary, renal and anal orifices open through a common 

 aperture on to the surface of the body. The loss of the shell thus 

 leads to the acquisition of a secondary symmetrical position of the 

 organs, a phenomenon that may also occur in other slug-like forms 

 (as also in various < >pisthobranchs). 



With regard to the further development of Onchidium, it need here 

 only be noted that the form of the adult is attained within the egg. 



The Vaginulidae, forms usually placed near to Onchidium, no 

 longer possess, according to Semper and v. Jhering, either the fully 

 developed bilobed velum or the larval shell (Xo. 116), although the 

 spawn appears to have the same constitution as that characteristic of 

 Onchidium (p. 104). These forms would therefore appear more 

 adapted to a terrestrial existence, if the short statements as to their 

 development should be corroborated. 



Onchidium and Vaijinulm are both opisthopneumonic, and this 

 fact, taken together with the other features of their organisation as 

 well as their ontogeny, suggests that they represent forms which, 

 from a condition like that of the marine Opisthobranchs, have become 

 adapted to a terrestrial existence. The classification of Onchidium 

 and Vayinidus among the Pulmonata which might, on account 

 of the peculiarities above mentioned, appear doubtful (Joyeux- 

 Laffuie), has been strengthened by the more recent observations on 



