382 PROCEEDINGS OF THE ACADEMY OF [April, 
There is yet another possible explanation of the renal organs as 
found in Opisthobranch larvee which will be stated but briefly, since a 
preponderance of hypothesis over fact is always to be regretted. It 
is generally conceded that whether the anal kidney be of mesodermal °* 
or ectodermal origin its rudiment is at first a paired structure, one 
part of which may fail to develop into a renal organ (Heymons) or 
unite with the other (Mazzarelli). The nephrocysts are paired struc- 
tures, one lying close to the anal kidney, the other in an almost similar 
position on the opposite side of the body. It is possible that the nephro- 
cyst of the right side is but a part of the anal kidney of that side, while 
that of the left represents the degenerate whole of the rudiment of that 
side. In this case, of course, true larval kidneys would be wanting. 
The Enteron. 
As the archenteron arises from the cleaving entoblast it presents, 
when viewed from the vegetative pole, an irregular depression, the 
bottom of which lies considerably below the edge of the blastopore. 
The macromeres, 5A, 5B, 5C and 4D, are at the bottom of this pit, with 
5a, 5b and 5e lying peripherally from them, while above these and next 
to the ectoblast come 4c?, 4b’, 4a? and the smaller cells 4c', 4b! and 4a'. 
In the posterior region are found the small cells E!, E?, e!, e? (entero- 
blasts) which have arisen from 4d. The fifth quartet and all the 
macromeres are the next cells to divide, this resulting in enlargement 
of the wall area of the enteron, and by this division into smaller ele- 
ments closer contact between the blastomeres results. Hitherto the 
entoblasts have been much rounded (except those meeting directly 
in the center), and have lain together in a very irregular manner, 
particularly after invagination began. With diminution in size and 
rearrangement of these cells a distinct cavity with closed dorsal wall 
arises (fig. 80). At the anterior end lies the large cell 4b?, while pos- 
teriorly and laterally are found the two large cells 4a”, 4c?; between 
and behind them are the enteroblasts. At first the enteron is longer 
on the right side (left of figures), the cell 4c? lying more posterior than 
4a, this being the natural result of the division which early separated 
the large mesentomere from 4D of that side and the lack of growth 
and division in this latter cell for so long a period. But as development 
proceeds and the whole enteron grows in antero-posterior extent it will 
be noted that 4a”, which is a very large cell and easily distinguishable, 
gains in its backward course upon the opposite cell of like lineage (4c?), 
comes to lie opposite to it and later more posterior (figs. 80, 81, 82). 
This process is the beginning of the torsion of the intestine, and is appa- 
