378 PROCEEDINGS OF THE ACADEMY OF [April, 



divide several times, one cell in each group remaining large. Thus 

 the excretory cells of Umbrella are ectodermal in origin. In further 

 history Heymons finds that the large cell of the left side decreases in 

 prominence and finally is indistinguishable from those surrounding 

 it, while the right continues to enlarge and, with the torsion of the larva, 

 is carried higher on that side. Later a second large cell appears by 

 the side of this one, which Heymons thinks cannot represent the 

 original left cell, as this would presuppose too great a migration, but 

 rather one of those associated with the original right, the growth of 

 which has been delayed. The function of a larval excretory organ is 

 assigned only to this group of cells by Heymons. 



In 1895 Mazzarelli, after a study of the development of a large num- 

 ber of forms {Philijie, Gastropteron, Actceon, Oscanius, Pleurohranchus, 

 Tethys, Archidoris, Aplysia, Hermcea, Janus, Polycera and Haminea), 

 came to the conclusion that the anal organ of Loven, Sars, Pruvot, 

 Lacaze-Duthiers and others was not, as Lacaze-Duthiers, Pruvot and 

 Heymons maintained, of ectodermal origin, but rather mesodermal, 

 arising from two large and other smaller mesoderm cells which become 

 pigmented and which by a slight ectodermal invagination acc{uire 

 an external opening. In later development he finds these cells form a 

 connection with the pericardium, which has arisen from a mesodermal 

 mass closely connected with them. Therefore, he concludes that 

 the anal kidney of the Opisthobranch larva is not homologous with 

 the head kidney of the Prosobranchs, but from its origin, position and 

 relation (particularly in connection with the pericardium) it is none 

 other than the anlage of the definitive kidney of the adult And 

 also, since it lies to the left of the rectum, it corresponds to the 

 kidney of the Gastropods which possess but one, and to the left kidney 

 of those with two. Viguier (1898) describes the anal kidney of Tethys, 

 distinguishing an excretory lumen, around which are grouped several 

 cells; he does not indicate its origin. 



Among the Prosobranchs externally situated larval excretory organs 

 appear to have been found generally. Salenskj^ (1872) has described 

 such bodies filled with concretions lying upon the side of the body in 

 CalyptrcBa and Nassa. Bobretzky (1877) found the same in Fusus, 

 these cells lying behind the velum and without an underlying ectoder- 

 mal layer. This latter condition is placed in doubt by McMurrich 

 (1886). Similar organs to the above were found in Fissurella by 

 Boutan (1885), while in Capulus (v. Erlanger, 1893) a single large 

 ectodermal cell, probably excretory in function, was found on each 

 side of the body behind the velum. For Crepidula Conklin (1897) has 



