THE DEVELOPMENT OF APLYSIA PUNCTATA. 531 



comparatively late stage become finally segregated out, in 

 the present case by proliferation. 



That the way in which organ-forming substances present 

 in the egg are finally separated from one another is quite 

 immaterial in affecting the homologies of the organs to which 

 they give I'ise is very clearly demonstrated by certain 

 experimental work, as, for example, that of Wilson on the 

 egg of Nereis. In this case the cleavage pattern was 

 totally changed by subjecting the egg to pi'essure, and yet 

 the larva produced was normal. 



These facts, taken together with what we know of the 

 movements of the cytoplasm before and during segmentation 

 in Cynthia, Dentalium, Cerebratulus, etc., show that the organ- 

 forming* substances often shift their position, and are segre- 

 gated at different periods. Meisenheimer's results have 

 demonstrated a remarkable instance of this, but provide no 

 evidence concerning the homologies of the organs. 



Before we discuss the larval excretory organs in Aplysia, 

 we may briefly describe the types found among Molluscs. 



I. Flame-Cells: 



a. The flame is borne by one cell only ; the duct is intra- 

 cellular. 



(1) Organ consists of two cells: Lamellibranchs. 



(2) Organ consists of four cells : Fresh-water Pul- 



monates and Basommatophora. 

 j3. The flames are borne by more than one cell; the duct 

 is inter-cellular: Terrestrial Pulmonates; Stylom- 

 matophora and Paludina. 



II. Ectoderm Cells which enlarge, become vacuolated 

 and project from the surface. 



a. Position variable, but near the base of the velum : 



Marine Prosobranchs. 

 /3. Position definite, slightly anterior to the anus: 



Opisthobranchs. 



III. Nephrocysts (primitive kidneys). A single cell some 

 distance beneath the surface, and without a duct : Opistho- 

 bi'anchs. 



