DEVELOPMENT OF THE EXTERNAL FORM PULMONATA. 



183 



now present. The respiratory cavity and the edge of the mantle 

 extend more to the left, the shell taking the same course. The head 

 becomes more distinct, rising up from the foot, which, in its turn has 

 increased considerably in size and has approached nearer its definitive 

 form. The velum has disappeared, a portion of it, according to 

 Ray Lankester, giving origin to the labial palps (p. 133). This 

 latter view seems quite in keeping with the position of the velum, 

 but is set aside as improbable by Fol and is directly refuted by 



WOLFSON. 



tiu 



y\m. 



Fig. 81. — Embryo of Helixpomatia seven days old. seen from the side (after Fol). ". 

 anus;/, toot: Kbl, cephalic vesicle; m, mouth; mil, enteron and digestive gland; 

 r, radular sac ; sd, shell-gland ; mi. primitive kidney. 



The shell is still cup-shaped, but is already asymmetrical. Further 

 unequal growth on one side leads to coiling both of the shell and the 

 visceral dome. 



<)ur account has, so far, referred chiefly to the development of the 

 freshwater Pulmonates, especially to that of a few forms which have 

 been particularly carefully investigated, such as IAmnaea and Plan- 

 vrUs. These latter have been described in detail by Ray Lankester 

 (No. 63), Rabl (No. 91), Fol (No. 33), and Wolfson (No. 131) to 

 whose descriptions we must refer the reader for further details. Fol 



