Bt Herdman on the " Olfactory TuhercUy 267 



The other varieties noticed differed first in shape, which 

 was crescentic, semicircular, or U shaped ; and, secondly, in 

 position, the tubercle being turned so that the aperture was 

 directed anteriorly, to the right side (Fig. 16), posteriorly, or 

 in any intermediate position, but never, in any specimen I 

 have examined, to the left side. 



The olfactory tubercle in this species is very constant in 

 shape, more so than in any other species of which I have 

 investigated a large number of specimens. 



I have no hesitation, therefore, in giving the following de- 

 scription as characteristic of the species : 



Olfactory tubercle small, simple, usually symmetrical and cres- 

 centic in shape. Aperture very wide, usually anterior, or 

 on the right side, rarely posterior. Horns not developed. 



These eight species of Simple Ascidians, in which I 

 have examined the form of the olfactory tubercle, represent 

 five genera belonging to the three great families of the Asci- 

 diidse, the Cynthiidse, and the Molgulidse. I think, therefore, 

 that I am fully warranted in considering that these species 

 may be taken as representative of, and my results as applying 

 to, at least the group of Ascidice Simplices. 



In no one of the species examined was the form and posi- 

 tion of the olfactory tubercle found constant, but the amount 

 of individual variation changed greatly according to the 

 species. The Molgida was the most constant species, while 

 Ascidia mrginea, and Ascidice pleheia were the most variable. 

 The points I wish to brin g out therefore are : 



1. That in Simple Ascidians the form of the "olfactory 



tubercle" may be different in individuals of the same 

 species. 



2. That the range of this individual variation changes ac- 



cording to the species. 



The result of this is, that the form of the olfactory tubercle 

 may be a characteristic feature in some species and not in 

 others, and I hold that whether it is a characteristic feature 

 or not for any particular species can only be determined by 

 the examination of a large number of specimens. 



