256 Proceedings of the Royal Physical Society. 



according to the family, genus, and species, the olfactory 

 tubercle also varies, and often very greatly, according to the 

 individual,* and that therefore great care must be exercised, 

 in using the condition of this organ as a specific character, 

 not to endow it with more than its proper value. 



In order to test the range of variation, and determine, if 

 possible, how far it is safe to trust to this organ as a specific 

 character, I have taken a few of the commoner species ot 

 Simple Ascidians, of which I had a sufficient number of 

 specimens to render my results accurate, and to justify me in 

 drawing conclusions as to typical forms, etc., and have ex- 

 amined the condition of the olfactory tubercle in a consider- 

 able number of individuals of each species. The chief results 

 of this investigation are the following : 



AsciDiA viRGiNEA, 0. F. MUUer. 



In this species the olfactory tubercle is of large size, and 

 extremely variable in form. The peri-tubercular area is small, 

 generally not accommodating more than one-third or one-half 

 of the tubercle ; as a result of this a considerable portion of it 

 lies in the zona prsebranchialis, and in many cases it extends 

 anteriorly to within a short distance of the tentacular circlet. 

 The peri-pharyngeal band as a rule extends considerably 

 higher up the tubercle on the left side than it does on the 

 right, thus rendering the peri-tubercular area unsymmetrical. 



The olfactory tubercle is elliptical or ovate in general 

 form ; the posterior end is broad and rounded, while the 

 anterior extremity shows endless variations in shape, con- 

 sequent upon the extent to which the ends or " horns " are 

 developed, and the manner in which they are curved. I have 

 only seen one specimen which was an exception to the rule 

 that the posterior end should be broad and regularly rounded ; 

 this individual (Fig. 1) had the lower (posterior) band con- 

 siderably thickened at the posterior end, so that this part 

 projected into the posterior angle of the peri-tubercular area. 



The simplest form of tubercle noticed in this species was 

 one in which the two horns at the anterior end were bent 

 slightly inwards so as to point towards each other with- 



* I fiud that Kupffer suspected this to be the case {loc. cit.). 



