1904.] NATURAL SCIENCES OF PHILADELPHIA. 381 



kidney and allied structures of Limax and Dreissensia, representing 

 two distinct molluscan groups, from ectodermal rudiments, after an 

 investigation which bears every evidence of care and accuracy, the 

 possibility at least of a similar manner of formation among the 

 Opisthobranchs must be granted. 



So little is as yet known of the "Nephrocysts" of Trinchese that any 

 discussion of their significance and possible homologies must of neces- 

 sity be largely hypothetical. An exact knowledge of their derivation 

 and structure would be of the utmost value. In Fiona when first seen 

 they lie in the cleavage cavity, but whether they have wandered there 

 from the ectoderm or are from the first mesodermal in character is 

 yet an imsolved problem. Should they prove to be of ectodermal 

 origin their position might justify a close homology with the Proso- 

 branch larval kidney, and possibly also with those of the Pulmonates 

 and Lamellibranchs, since Meissenheimer has indicated the larval 

 kidneys of the two latter groups to be of ectodermal origin, and his 

 work is supported bj' the earlier investigations of Wolf son and Fol. 

 Should these nephrocysts prove entirely mesodermal there is yet a 

 possibility of their similarity to the larval kidneys of the Prosobranchs, 

 Lamellibranchs and Pulmonates, through the investigations of Biitschli 

 and Erlanger for the Prosobranchs, Rabl and Holmes for the Pulmo- 

 nates and Hatschek for the Lamellibranchs, who derived the primitive 

 kidney of members of these groups in part or entirely from mesodermal 

 elements. However, the structure of the nephrocj'sts of Opistho- 

 branchs is very different from the primitive renal organs of the groups 

 above cited, for, as far as is known, they appear wholly enclosed in 

 the schizocoel with no external ducts. The fact of their very rudi- 

 mentary structure suggests an explanation for the great development 

 reached by the anal kidney. When we consider that in other groups 

 possessing true larval excretory organs the anlage of the definitive 

 kidney does not develop into a condition of functional activity until 

 after metamorphosis, while among Opisthobranch larvae, even before 

 the time of hatching, certain cells of this structure are actively con- 

 cerned in the work of excretion, the causal relation between rudimen- 

 tary structures on the one hand and advanced development on the 

 other is brought forcibly to mind. The nephrocyst of the Opistho- 

 branch is not a prominent or well-developed structure, and with its 

 phylogenetic decline precocious development has arisen in the rudi- 

 ment of the definitive kidney, resulting in functional activity in a 

 part at least of its formative elements long before development of 

 the adult organ. 



