ON THE DEVELOPMENT OF THE POND-SNAIL. 371 



in these animals the shell is from the first formed in a sac. 

 In fact, we should only have to retain the shell -gland of the 

 allied pulmonate Lymnceus, in adult life, in order to produce 

 precisely the required internal shell of Limax. It seems, 

 therefore, very probable that the shell of Limax is identical 

 with the plug of the shell-gland, which has so wide a dis- 

 tribution among the embryos of MoUusca. At the same time 

 further knowledge of the development of Limax and other 

 Pulmonata, is necessary for a satisfactory conclusion on this 

 point. 



The further question as to the identity of the shell-gland 

 and its plug with the pen-sac and pen of the dibranchiate 

 Cephalopods is of very great importance and great difficulty. 

 Professor Gegenbaur, in his ' Grundz. der vergleich. Ana- 

 tomie,' puts forward the view that the Cephalopoda, on 

 account of their bilateral symmetry and general anatomical 

 relations with the other Mollusca, are to be regarded as the 

 least specialised group of the whole stock ; that is, as more 

 closely retaining the characters of the common ancestors of 

 existing Mollusca than do any other forms. If this be so, 

 we should expect to find a representative of the shell-gland 

 in the organization of the Cephalopods, and our attention is 

 immediately directed to the pen-sac and pen of the Dibran- 

 chiata. If the shell-gland and pen-sac are identical structures 

 we have a brilliant confirmation of Gegenbaur's view. This 

 was one of the chief matters to which I directed my attention 

 in a recent study of the development of Loligo, Sepiola, 

 Sepia, and Octopus. I was anxious to determine the exact 

 mode of the first commencement of the sac in which the 

 " pen" of these cuttle-fish develops. I have only space here 

 to state that it makes its first appearance as a relatively very 

 small circular pit, the sides of which close in above so as to 

 form a shut sac, which enlarges and elongates with the later 

 growth of the embryo. In fig. 3 is given a drawing of a sec- 

 tion of a very young embryo (the drawing is cut off so as to 

 omit the yelk-sac and give only the embryonic portion of the 

 specimen) at a stage when the pen-sac is still open, and 

 its lips commencing to close in. Its position and mode of 

 development exactly agree with that of the shell-gland as 

 seen in the other molluscan embryos figured in this paper. 

 We are, therefore, fairly entitled to conclude, from the em- 

 bryological evidence, that the pen-sac of Cephalopoda is 

 identical with the shell-gland of other Mollusca. 



But here — forming an interesting example of the inter- 

 action of the various sources of evidence in genealogical 

 biology — palaeontology crosses the path of embryology. I 



VOL. XIV. NEW SEtt B B 



