284 PKOF. W. A. HERDMAN ON 



In A. 7nentula the usual characters o£ the branchial sac seem to be as 

 follows : — There is well-marked " plication " o£ the wall, intermediate papillae 

 are always present on the internal bars halfway between the transverse 

 vessels, the stigmata are wide and short and vary from 5 to 16 in a mesh — 

 the usual numbers being from 9 to 12. 



In A. venosa the plication of the wall is much less marked, there are no 

 intermediate papilla? (except occasionally where a range of stigmata is in 

 process of dividing into two — and in that case the papillse are not really 

 "intermediate" as they will eventually be at the angles of meshes), the 

 stigmata are longer and narrower and more regular, and vary in number 

 from 7 to 15 in a mesh, the usual numbers being from 8 to 12. 



If we now attempt to draw some conclusions from these studies of the 

 variation of the internal organs which are considered of most importance in 

 defining the species, they must be to this effect : — Ascidia mentula and 

 A. venosa can always be easily distinguished from one another by the external 

 appearance either alive or when preserved. They may also be readily 

 distinouished when stripped of the test. In the hundred or more specimens 

 which we have just passed through our hands there has never been the least 

 doubt as to which species each individual belonged to. Of the Uyo, A. mentula 

 is the more variable. Then, again, when examining the microscopic anatomy 

 of the internal organs, such as the branchial sac, there is a genei-al facies 

 which is found to be characteristic <>f the organ in each of the species and 

 which in most cases enables one to name the species correctly. But it is 

 important to note that in these organs the range of individual variation in 

 what are usually regarded as specific characters may, as we have seen, be 

 very wide, and the extremes of the one species may reach to or even overlap 

 those of the other. Consequently, it might be difficult in the case of a given 

 preparation of an organ, say branchial sac or tentacles, placed under the 

 microscope to determine the species with certainty, but even in that case if 

 the other internal organs are available for examination, from the combination 

 of characters there ought to be no difficulty in making a correct iden- 

 tification. 



These two species which I have chosen out first for the purpose of such a 

 comparison of their characteristics are very distinct from one another — so 

 much so, in fact, that they are sometimes placed in separate but neighbouring 

 genera, Ascidia and Ascidiella*. I am afraid, when one comes to deal in 

 this manner with the variations in other more closely related so-called species 

 of Ascidia, it will not prove such an easy matter to discriminate and define. 

 There is no doubt that such investigations ought to be made whenever large 

 series of individuals can be obtained, and I feel pretty sure that the work 



* Separated off mainly on account of the relative positions of the nei've-gangiiou and the 

 dorsal tubercle. 



