3S0 PROCEEDINGS OF THE ACADEMY OF [April, 



dermal invaginating tube with the end closed by a vacuolated heavily 

 cihated cell. 



From 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 

 jjarticLilarly of the Opisthobranchs), one is strongly impressed with the 

 feeling that much more work must be done upon these organs of mul- 

 luscan larvse 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, Ijut its derivation is totally 

 different from the results obtained by some of the more recent and 

 careful workers in this group. 



Mazzarelli's conclusions 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 fimctional 

 condition upon the right side of the veliger larva at the time of hatching. 

 In this respect my results are entireh^ 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 a 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 



