THE DEVELOPMENT OF APLYSIA PUNCTATA. 513 



These four cells become differentiated while in the ecto- 

 dermal layer on the right side of the embryo and slightly in 

 front of the anal cells. Even in the earliest stages, which 

 we have examined, their nuclei are cleai-ly to be distinguished 

 from all the other nuclei in the embryo, not only by their 

 much larger size, but by the presence in almost every case, 

 of a conspicuous deeply-staining plasmosome. Text-tig, 15 is 

 a section of a stage where these cells have just begun to sink 

 below the surface. Of the two cells shown one is still in the 

 outer ectodermal layer, while the other has already sunk 

 below. This process has gone further in Text-fig. 16 and the 

 four cells are covered by a thin ectodermal layer. They con- 

 tinue to sink in further, and gradually give rise to a compact 

 pear-shaped organ, the apex of which is directed towards the 

 surface. Text-fig. 17 is a section taken at a stage when the 

 organ is first becoming definite. We have never seen any of 

 the four cells in the process of division ; but the kidney in the 

 free-swimming larva consists of eight cells, and therefore each 

 original cell must divide once. The cytoplasm is at first 

 finely vacuolated, but as development proceeds the small 

 vacuoles become confluent, and form in the external part 

 of the kidney several large cavities, the narrow ends.of which 

 converge to a point where they open into the mantle cavity. 

 Here two small ectodermal cells form a short duct (Text-fig. 

 19). In the living larva drops of coloured liquid are seen to be 

 contained within, the vacuoles, but as a rule they are dissolved 

 oat by the reagents used in the coui-se of preservation. The 

 whole organ is clothed with a thin mesodermal epithelium. 



This single excretory organ has long been known in 

 Opisthobranch larvae, but the most diverse statements have 

 been made both as to its origin and function. With regard to 

 the latter point, there can be no doubt that it is an excretory 

 organ since it is easy in the living larvge of some Opistho- 

 branchs to observe the process of excretion. The view put 

 forward by Lacaze-Duthiers and Pruvot (13) that it was an 

 " anal eye " must be held to be one of the most curious and 

 unwarranted of zoological speculations. Two investigators. 



