66 bulletin: museum of compaeative zoology. 



and the loop of the digestive tract occupies the sagittal plane. By 

 means of this twisting, the anus becomes dorsal and the point of union 

 of the genital strand with the perihranchial sacs migrates from the 

 left through the cloaca to the right perihranchial sac; and thus the 

 adult relations of the organs are acquired. 



Conclusions. 



From this account it follows that the development of the sexual 

 organs in Distaplia occidentalis agrees with that of the three species 

 described by van Beneden et Julin ('85, pp. 328-349) and of Ciona 

 intestinalis described by Floderus ('96, pp. 173-181) in the following 

 important respects : — 



1. The whole of the sexual organs of the adult is developed from a 

 single solid mass of cells of mesodermic origin, which during the later 

 period of its existence is connected with the cloaca by a genital strand. 



2. The ovary is separated from the testis by a splitting of the primi- 

 tively single fundament which proceeds from behind forward. The 

 separation of the two ducts takes place in the same manner, though 

 here growth must intervene. The ovary and oviduct always lie on the 

 superficial side of the testis and vas deferens. 



3. The cavity of the ovarial fundament is formed in such a way that 

 the germinative epithelium lies in its deeper wall. 



Distaplia occidentalis differs from the species mentioned in that : — 



1. The ovary and testis are usually solid, and the ducts always so, 

 at the time when they are separated from each other. 



2. The genital strand is at first quite thick, and decreases in 

 diameter as the bud grows. 



3. The fundament of the ovotestis is present in the youngest stages, 

 whereas in the other species it appears quite late in ontogeny. 



With the development of these organs in Styelopsis grossularia 

 studied by Julin ('93), that of our species has less in common, because 

 in the two the ovaries and testes are of a different type. In Distaplia, 

 and the four species with which it has been compared, the ovary is 

 distinctly separated from the oviduct proper, in the walls of which no 

 ova arise, while in Styelopsis the deep wall is given over to the pro- 

 duction of ova throughout the whole extent of the oviduct. Further- 

 more, in Styelopsis the testis is divided into a number of lobes, each 

 one opening separately into the perihranchial cavity, whereas in the 

 other species it is a single organ with a single long duct. Accord- 



