514 A. M. CAKR SAUNDERS AND MARGARET PDOLE, 



Casteel working with Fiona and Heymons with UmV>rella, 

 have come to the same conchisions as we have regarding 

 the origin of this organ. In Umbrella it is originally paired. 

 There are large ectodei'mal cells, 3r'i and 'ScV\ on either side 

 of the embryo which divide and sink below the surface, one 

 cell in each group remaining especially large. Tlie cells 

 on the left later disappear, while the right group forms the 

 kidney. In Fiona the secondary kidney is unpaired from 

 the beginning, as it is in Aplysia. It consists, however, of 

 a single cell, 3r^'^'. This closely resembles the cells which 

 form the organ which we have described in Aplysia, the 

 cytoplasm being much vacuolated, and the nucleus large 

 and containing nucleoli. In this case there are also other 

 ectoderm cells near by, which seem to function in the same 

 way. Clearly, we are dealing with a very similar organ 

 in these three forms, but in Aplysia it is better developed, 

 forming a definite organ with a duct and an enveloping 

 epithelium. 



The ectodermal origin of this kidney was first recognised 

 by Lacaze-Duthiers and Pruvot. Mazzarelli is the only 

 recent writer who upholds the vicAv that it is mesodermal. He 

 has worked on Aplysia and a number of other Opistho- 

 branchs, and has come to the same conclusion for them all. 

 We find his observations difficult to reconcile with our own. 

 The organ in question, according to his account, is derived 

 from two mesodermal cells at the aboral pole, which repre- 

 sent a paii-ed rudiment of the kidney, as in Umbrella ; in 

 the course of torsion, however, both cells get pushed round 

 on to the right side and form the single unpaired structure. 

 They divide, become surrounded by other smaller meso- 

 dermal cells, and finally come to communicate with the 

 exterior by an ectodermal invagination. It would seem that 

 he took for the rudiment of the kidney two of the large 

 mesoderm cells, which lie, at the stage he describes, on either 

 side of the aboral pole : the large ectodermal cells, still 

 lying at the surface, he has apparently overlooked. But 

 why at a later stage he should describe two cells, when 



