THE FORMATION OF THE ORGANS — THE SHELL. 187 



doreally towards the viscera which are surrounded by a blood-sinus. 

 \ provisional circulation thus exists side by side with the definitive 

 circulation. 



Towards the end of embryonic life, the pedal vesicle decreases in 

 size. It remains at first as an appendage to the foot, but this vestige 

 also disappears, being absorbed. The foot thus assumes its final 

 shape. A median invagination, which only appears at a late stage 

 on the foot near the mouth and lengthens out posteriorly into a tube, 

 represents the rudiment of the pedal gland (Fol). 



Apart from the development of these embryonic organs which are 

 here specially large, the further development of the embryo resembles 

 that of other Gastropods and especially of the aquatic Pulmonata. 

 'Phis also applies to the shell where this is not vestigial and internal 

 as in many terrestrial Gastropods. Where there is an internal shell, 

 as in Limax and Arion, the shell-gland becomes disconnected from 

 the ectoderm, as already explained. The shell remains internal, being 

 hidden beneath the mantle, and is a vestigial structure. In Arion it 

 consists merely of a number of disconnected calcareous granules. 



It is a striking fact that, in ( 'lausilia, according to Gegenbaur, the shell 

 also at first lies internally enclosed in the epithelium of the shell-gland. 

 Only when this latter, as well as the mantle-tissue above it, disappears, does 

 this internal shell become external, develop and become coiled. As far as 

 we know, this somewhat inexplicable observation of Gegenbaur has not been 

 corroborated.* We feel inclined to explain the phenomenon described on 

 the belief that there is retained a small aperture over the shell as it lies 

 within the shell-gland, this gland flattening out at an unusually late stage. 



6. The Formation of the Organs. 



A. The Shell. 



We have already, in treating of the external shape of the body, 

 repeatedly alluded to that of the shell, so that only a few further 

 remarks need be added. The shell arises from the shell-gland, and, 

 when the latter has flattened out, appears cap-like. At first, there- 

 fore, there is great resemblance in this point to the Lamellibranchs. 

 Here also a shell-integument forms first, beneath which the calcareous 

 substance is deposited later. The further processes are altogether like 

 those in the Lamellibranchs as given more in detail on p. 60. The 



* [Schmidt (No. XXI.) has since confirmed Gegenbaur's observations that 

 the shell-gland closes completely and later opens out again both in Clausilia 

 and Succinea. — Ed.] 



