OiN THK UKVELOPMENT OF THE POND-SNAIL. 383 



I have no details to record Avith regard to the develop- 

 ment of the eyes ; but with regard to the otocysts may 

 draw attention to the fact that they are absent in the stages 

 studied by me, though in a corresponding period of develop- 

 ment in the Nudibranchiata they have attained a high degree 

 of perfection. This I imagine may be explained by the 

 relatively smaller importance of the auditory organ in the 

 adult Lyninceus than in the Sea-slugs^ and their near allies 

 the free-swimming snails (Heteropoda). 



Mantle-jiap andlung. — In PI. XVII, fig. 8, a stage in which 

 the shell-gland has disappeared, and the shell itself {sh) is 

 already projecting like a watch-glass from the aboral pole of 

 the embryo, the edge of the mantle first becomes raised up and 

 definitely omarginated. Following this through figs. 7 and 

 10, we find its rim becoming more and more detached or 

 lengthened, until in fig. 18, on the right-hand side, a con- 

 siderable space is overhung by this marginal flap. It is here 

 that the lung develops as a simple recess covered in by the 

 mantle-flap. In the specimen drawn in fig. 18 the rudi- 

 ment of the heart is also seen (A), and other organs in con- 

 nection with the enlarged border of the mantle, viz. the 

 tubular dark-coloured body opening to the exterior {n), which 

 I take to be the young kidney, and the prolonged delicate 

 terminal portion of the alimentary canal, which still ends 

 blindly (cr). 



Alimentary canal. — The fact that the alimentary canal 

 ends blindly in so late a stage of development as that of 

 fig. 18 should have made clear to M. Lereboullet that he was 

 wrong in interpreting the shell-gland as an anal cone ; but 

 it must be admitted that to follow out fully the development 

 of the alimentary canal is exceedingly difficult, even as far as 

 its general contour is concerned, still more so when a his- 

 tological and histogenetic point of view is attempted. In 

 fact, here, as iu all the embryologies which have been 

 attempted, the dark point is in connection with the middle 

 portion of the alimentary canal. If we knew with certainty 

 whence and how its cellular elements are developed in all 

 types which have been studied, we should have little diffi- 

 culty in reducing the facts of development of the whole 

 animal kingdom to satisfactory order. 



We have seen that there results from the gastrula-invagina- 

 tion an outer cellular body- wall, from the elements of which 

 the epidermic and muscular structures of foot, velum, mantle, 

 and shell-gland, develop, and an inner invaginated sac com- 

 posed of larger cells, supported on a short pedicle (the cells 



