382 PROCEEDINGS OF THE ACADEMY OP [April, 



There is yet another possil^le explanation of the renal organs as 

 found in Opisthobranch larvse 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 cntoblast 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, 5 A, 5B, 5C and 4D, are at the bottom of this pit, with 

 5a, 5b and 5c lying peripherally from them, while above these and next 

 to the ectoblast come 4c', 4b^ 4a2 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. SO). At the anterior end lies the large cell 4b2, while pos- 

 teriorly and laterally are found the two large cells 4:0?, 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- 



