THE DEVELOPMENT OF APLYSIA PUNC'I'ATA. 529 



examples will, however, serve to show that a well-developed 

 coelora is of frequent occurrence in the Mollusca, and that it is 

 probable that when the later stages in other Molluscan groups 

 have been more thoroughly examined, a large coelom like 

 that which we have described in Aplysia will be found to be a 

 normal feature in the organisation of the Molluscan larvas. 

 But it is to be observed that the coelom in Aplysia is developed 

 at a stasre when in both Annelids and Molluscs the mesoderm 

 bands are still intactand a cavity has not yet been developed. 



In very few forms among the Mollusca has the development 

 of the coelom been traced from the segmentation period 

 onwards. Among the forms which have been worked out, 

 Aplysia and Physa follow what we may call the normal 

 Annelid type, that is to say, the mesoderm, all of it in 

 Aplysia, and the greater part of it in Physa, is developed 

 from 4d, and from it the coelom arises. In the others there 

 is a departure from this type of development ; there are 

 Dreissensia, Limax, and Cyclas, which have been described by 

 Meisenheimer, and Paludina, according to Otto and Tonniger. 

 In the first three cases the cell lineage is known, and 4d 

 develops in the usual way and gives rise to bands, which split 

 up and form mesenchymatous tissue, and thus seems to corre- 

 spond to the larval mesoderm described by Lillie in the 

 Unionidse, where it arises from 2a^, but is believed to give 

 rise to the adductor muscle. The coelom is stated to arise, 

 not from the descendants of 4d, but from cells which proliferate 

 from the " ectoderm " at a comparatively late stage when 

 segmentation is complete. The same is said to be the case 

 with Paludina; the cell lineage is not known in this form, but 

 it is distinctly stated that there are no pole mesoderm cells. 



Such a marked departure from the typical mode of develop- 

 ment was hardly to be looked for; it is to be noticed that it 

 occurs in widely separated members of the phylum, and 

 further, that there are no peculiar bionomic conditions 

 common to them. So far as our knowledge goes, it would 

 seem to be an alternative mode of development, which may 

 occur anywhere in the Mollusca. At fli'st sight it might 



