DEVELOPMENT OF FLUSTKELLA HISPIDA. 469 



formation of an archigastrula with a definite opening to the 

 exterior : this communication becomes obliterated at a later 

 stage. The fully-developed larva has a stomach which com- 

 municates with the exterior, similar to that which has been 

 described by Harmer (11) as present in the developing larva 

 of Alcyonidium. 



Prouho (21) describes the presence of a functional ali- 

 mentary canal in the " Cyphonautes " larva, the stomach 

 being developed from the internal endodermic mass in the 

 manner described above. In an earlier paper on Flustrella 

 hispida (20), he regards the pharynx as a rudimentary 

 digestive tube less differentiated than that of Alcyonidium, 

 and representing either an attempt to form a digestive organ 

 or a vestige of one which has vanished. 



Harmer (11) found, in a species of Alcyonidium, a 

 definite alimentary canal closely resembling that occurring 

 in Flustrella hispida, but communicating by a narrow 

 opening with the exterior. In the same paper he points out 

 the probability that a similar structure occurs in Flustrella 

 hispida. 



From its endodermic origin, its position, appearance, and 

 mode of development, and from the close agreement in 

 structure with the stomach described in Alcyonidium, there 

 can be no doubt that the larva of Flustrella hispida 

 possesses a vestigial stomach, and that this and the associated 

 pharynx must be regarded as vestiges of a digestive system 

 in which degeneration has proceeded a stage further than it 

 has in Alcyonidium, since no communication with the 

 exterior is ever established at any stage. 



General Summary. 



The main points in the foregoing paper may be summarised 

 as follows : — 



(1) A "yolk nucleus " of the type described by Bambeke_, 

 as occurring in the egg of P hole us, is present in the de- 

 veloping egg of Flustrella hispida. 



