380 PROCEEDINGS OF THE ACADEMY OF [April, 
dermal invaginating tube with the end closed by a vacuolated heavily 
ciliated cell. 
I’rom the above account of some of the more important observations 
and conclusions upon the nature and origin of the larval excretory 
organs of the Lamellibranchs and Gastropods (and of the latter more 
particularly of the Opisthobranchs), one is strongly impressed with the 
feeling that much more work must be done upon these organs of mul- 
lusean larvee before we are ready to come to definite conclusions 
regarding their mutual relations and homologies, if such exist. Nor 
has the investigation recorded in this paper brought forward facts 
which justify an immediate solution of the problem. The anal kidney 
of Fiona doubtless corresponds to the similar structure described for 
so many members of the Opisthobranchia, but its derivation is totally 
different from the results obtained by some of the more recent and 
careful workers in this group. 
Mazzarelli’s conelusions regarding its mesodermal origin, resulting 
from investigations upon a large number of closely related forms, are 
very different from mine. There is no point regarding the cytogeny 
of Fiona of which I am more certain than that the group of cells con- 
stituting the anal kidney is of ectodermal origin, and one member 
of the group (the largest, 3c) has been traced through every: step 
of its history, from the initial cleavages which produce it to its functional 
condition upon the right side of the veliger larva at the time of hatching. 
In this respect my results are entirely in accord with those of Heymons 
for Umbrella and, except for the function assigned to the resulting 
organ, agree closely with Lacaze-Duthiers and Pruvot’s derivation 
of the same structure from ectodermal cells. With regard to the 
fate of this organ, the work of Rho and Mazzarelli appears to show con- 
clusively that it becomes metamorphosed into the kidney of the adult, 
and the latter’s comparison of this organ with the adult kidney of 
those Gastropods which possess but one, or with the left of those with 
two, is in entire accord with the generally accepted opinion upon this 
subject. Unfortunately material has not been available for a study 
of the metamorphosis of Fiona. But on @ priori grounds it should 
be similar in all essential features to the above-mentioned processes of 
development in closely allied forms. The metamorphosis of the anal 
kidney of the larval Opisthobranch into the definitive kidney of the 
adult might seem, at first sight, fair grounds on which to doubt its 
ectodermal origin, since the latter structure has generally been con- 
sidered to be a mesodermal derivative. But if in this connection be 
considered the recent results of Meissenheimer, who derives the adult 
