580 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



they are ciliated with exterior apertures. These have been found 

 in Bythima and Paludina (BiitschH, 1877, Erlanger, 1891a, 1892&). 



{b) Opisthohranch Gasteropods. — Here there are distinguished 

 nephrocysts and anal kidneys. The nephrocysts were discovered 

 and named by Trinchese (1881) for Ercolania, Aniphorina, Bergia 

 and Doto; and were described also by MazzarelH (1892) for Aplysia 

 and by Casteel (1904) for Fiona. These are rounded bodies lying 

 anterior to the anus in the blastocoel, without external ducts ; nothing 

 positive is known of their origin, and Trinchese supposes them 

 mesoblastic simply from their position. They may occur in the 

 same embryo together with the following organs. The anal kidneys 

 were first interpreted as excretory by Langerhans (1873, Doris and 

 Acera). They are a pair of single cells, or groups of cells, that 

 originate near the anus but may migrate further forward. Trinchese 

 (1881) and Guiart (1901) derived them from the mesoblast, and 

 so also did MazzarelH {Aplysia, 1892, 1898) who ascribed the occa- 

 sional unpaired condition to the fusion of a pair. But Lacaze- 

 Duthiers and Pruvot (1887) described them as ectoblastic, and this 

 conclusion was reached also in the careful studies of Heymons 

 (1893, Umbrella) and Casteel (1904, Fiona). Casteel's work is the 

 most thorough on any opisthohranch, and he states : " There is no 

 point regarding the cytogeny of Fiona of which I am more certain 

 than that the group of cells constituting the anal kidney is of ecto- 

 dermal origin." 



(c) Pulmonate Gasteropods. — Here again there are two kinds 

 of larval kidneys. The external kidneys (aussere Nieren) occur 

 one on either side of the body, each a projecting group of vacuo- 

 lated cells forming part of the ectoblastic velum. These were dis- 

 covered by Biitschh (1877), and have been described by Fol (1880) 

 and Rabl (1879) for Planorbis. Much more attention has been 

 given to the head kidneys (Urnieren). The most detailed descrip- 

 tion of these in their perfected condition is that of Meisenheimer 

 (1898, 1899) : in the Basommatophora (Ancylus, Physa, Planorbis. 

 Limncra) these are much alike, each consisting of but four cells 

 with intracellular cavity, the innermost of which closes the canal 

 against the blastocoel and bears a ciliary flame. In the Stylom- 

 matophora (Limax, Snccinea, Helix, virion) the cells are much 



