DEVELOPMENT OF THE EXTERNAL FORM — PULMONATA. 



177 



later, secreting the shell in the usual way ; in Li max, however, the 

 shell of which is at first internal, the shell-gland is pouch-like and 

 becomes abstrieted from the ectoderm (Fol). A swelling of the body 

 behind the month indicates the position of the foot (Fig. 78). The 

 velum appears in the form of two transverse swellings (formed of 

 large, richly vacuolated cells) in front of the mouth, which run as 

 bands round a large part of the anterior body, but for a time do 

 not meet, or else, as in Planorbis, in consequence of the very much 

 reduced condition of the velum, never completely unite (Fig. 78, v). 

 At this stage, we may, with Ray Lankester, consider the embryo 

 as equivalent to the Trochophore ; occasionally, as in Llmnaea, even 



Fig. 78. — Planorbis embryo, seen from the side (after Rabl). air, eye; m, mouth; 

 md, enteron and digestive gland (large cells); mes, mesoderm; ot, otocyst; r, 

 radular sac: s, shell: sd, shell-gland; sp, apical plate: wti, primitive kidney; v. 

 velum. 



the external form of the Trochophore is preserved, a large prc-oral 

 portion of the body being marked off from the posterior portion b}- 

 the velum (Bay. Lankester, Fol). A thickening at the pre-oral 

 pole denotes the apical plate. That the bilobed character of the 

 velum so characteristic of the 1 r eliger larva is found here also is due 

 to its mode of origin. As a rule, not only the Veliger stage but the 

 Trochophore stage as well is much reduced, the principal features of 

 the latter, however, are still to be found. 



At the stage which more or less corresponds to the Trochophore, 

 the alimentary canal consists of a stomodaeum from which a radular 

 sac soon grows out ventrally (Fig 78, r) and the still undivided and 



N 



