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 (Philine, Gastropteron, Acteon, Oscanius, Pleurobranchus, 
Tethys, Archidoris, Aplysia, Hermea, Janus, Polycera and Haminea), 
came to the conclusion that the anal organ of Lovén, 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 acquire 
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. Salensky (1872) has described 
such bodies filled with concretions lying upon the side of the body in 
Calyptrea 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 MeMurrich 
(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 
