1904.] NATURAL SCIENCES OF PHILADELPHIA. 381 
kidney and allied structures of Limax and Dreissensia, representing 
two distinct mollusean 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 unsolved 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 by the earlier investigations of Wolfson 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 Bitschli 
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 nephrocysts 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 larve, 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. 
