ON THE DEVELOPMENT OF THE POND-SNAIL. 381 



prominent position, and pushing its way right up into the 

 central mass of gastrula-endoderm-cells, it subsequently 

 dwindles, and very rapidly disappears altogether, as the shell 

 forms and the mantle-area becomes raised up as a convex dome 

 with margin distinctly projecting to form a rudimentary mantle- 

 flap. The drawings (PI. XVII, figs. 11, 12)represent the shell- 

 gland in its most strongly marked condition, the conical 

 lumen of the gland being filled by a highly refracting chiti- 

 nous substance. Curiously enough, the cases in which this 

 occurs appear to be abnormal. In a mass of eggs which 

 have for the most part advanced to the stage seen in PI. XVII, 

 fig. 10, two or three may be found which have hung back, and 

 have an abnormal proportion of foot, mantle-flap, &c., besides 

 being much smaller than the further advanced normal speci- 

 mens. Such retarded specimens frequently exhibit the con- 

 dition of the shell-gland figured in figs. 1 and 12. Not 

 only is there this plugging of the gland, but the commencing 

 shell (not at this period calcified, but entirely of a horny 

 composition) is thick and rough as compared with the 

 normal shell of the same size, and sometimes the plug is 

 united to the disc-like shell, so that the two can be picked 

 out by careful teazing as a separate plate and handle. In 

 the introductory remarks above I have referred to my obser- 

 vations on Ajilysia [Pleurohranchidium), where I found 

 a precisely similar condition accompanying a retarded 

 development. 



TJie Velar Area. — Let us now return to the velum for the 

 purpose of tracing its development, and that of the velar area. 

 It is an extraordinary fact that the existence of a velum in the 

 embryos of Pulmonata has been denied, and its absence is at 

 this moment mentioned in so authoritative a work as Bronn's 

 ' Thierreich ' as characterising the young stages of that 

 group. Pouchet, who appears to have seen it in the trocho- 

 sphere-phase of Lymn(Bus, and whose figures are copied in 

 Bronn's plates, traces it to the free edge of the mantle, for 

 the first rudiment of which, he mistakes it. LerebouUet 

 appears to have missed it altogether. The fact is, as will be 

 seen from the figures in PI. XVII, that it not only is well 

 developed in the youngest stages of LymncBus, but persists in 

 an altogether exceptional way, and is actually retained in the 

 adult, having become the lip-like masses which are known 

 in Lymnceus as the " subtentacular lobes." The margin of 

 the velum is easy to trace in the Veliger-phase of Lymnceus 

 on account of the large, granular, epidermic cells of a 

 yellowish-brown colour which compose it. When the em- 

 bryo passes from the Veliger-phase to the definite nioUuscan 



