Miscellaneous. 375 



On the Development of the Pulmonate Gasteropoda. 

 By M. H. Fol. 



Although this group of Mollusca has already been the subject 

 of numerous works, the following liues will show how incomplete 

 and erroneous are still the notions that we possess as to their de- 

 velopment. 



Segmentation takes place in a manner conformable to what is 

 observed in the Heteropoda. In all there is total segmentation, 

 leading to the formation of a blastosphere, the nutritive half of 

 which, consisting of elements of larger size and richer in proto- 

 lecith, becomes invaginated in the other half. The aperture of in- 

 vagination is nothing but the primitive mouth, and certainly does 

 not become the anus as Mr. E. Ray Lankester asserts. It occupies at 

 first the nutritive pole — that is to say, the pole opposite to the polar 

 corpuscles (corpuscules de rebut) ; but these two poles soon become 

 displaced, in consequence of the more rapid development of the 

 ventral half of the embryo, in which the foot and the precoiichylian 

 invagination originate. The dorsal side of the primitive mouth is 

 surmounted by a projection, which is particularly developed in 

 HelLv, a projection which enters by degrees into the oesophagus, to 

 form there a winged longitudinal crest which afterwards disappears. 

 This crest, which M. Lhering compares to the velum of other Gaste- 

 ropoda, has really nothing in common with that organ, and may be 

 related to the analogous projection that I have described in the 

 embryos of the Pteropoda. 



The cells of the formative pole secrete between them a liquid 

 which finally detaches all this region of the ectoderm and separates 

 it from the entoderm. The vesicle thus produced occupies, in the 

 aquatic Pulmonata, only the bottom of the dorsal region in the 

 neighbourhood of the shell-depression ; in the terrestrial Pulmonata 

 this vesicle occupies the whole of the dorsal region as far as the 

 mouth, and attains considerable dimensions, only diminishing at the 

 moment when the pedal sinuses begin to dilate. 



The formation of the digestive tube is the same as in the Hetero- 

 poda. The embryonic digestive cavity is filled only with albumen, 

 and not with a compact cellular tissue, as stated by M. Rahl. It 

 never ceases to communicate with the exterior by the ciliated canal 

 of the primitive invagination ; only this canal buries itself more 

 deeply at the same time with the neighbouring ectodermic tissues 

 which form the oesophagus and the sac of the radula. The salivary 

 glands are evaginations of the wall of the oesophagus on the sides 

 of the sac of the radula. The deutolecith accumulates in great 

 abundance in a portion of the cells of the embryonic digestive cavity, 

 r.nd forms one nutritive lobe or sac in the terrestrial Pulmonata, two 

 lobes in the aquatic forms. These lobes are directly converted into 

 the liver after the absorption of the deutolecith that they contain ; 

 the hepatic cells are the entodermic cells of the nutritive sac, and 

 not mesodermic cells as Mr. Lankester supposes. The intestine and 

 the anus are formed as in the Heteropodn 



