282 PEOF. W. A. HERDMAN ON 



Ascidia mentula and A. venosa* are easily distinguished by their external 

 appearance. Amongst more than a hundred specimens collected and pre- 

 served there was not a single doubtful case. Although individuals vary to 

 some extent, especially in A. mentula, still every specimen can at a glance 

 be referred to one or the other species. 



I had removed a number of the freshly-dredged specimens of both species 

 from their tests, thinking that this would ensure better preservation ; but 

 examination of the material now shows that this was a mistake, as these 

 "skinned'^ specimens are so contracted and distorted as to be much less 

 suitable for investigation than those specimens where the test was left as a 

 support to the more delicate tissues within. 



We t picked out ten well-preserved specimens of each species and made 

 microscope preparations of the wall of the branchial sac, of the entire series 

 of branchial tentacles, and of the dorsal tubercles, with the following 

 results : — The specimens of Ascidia mentula ranged in length (antero- 

 posterior) from 12 to 16 cm. The tentacles were found to vary in number 

 from 39 to 103, and the number seems to bear no relation to the size of the 

 body, since the individual with only 39 was actually larger than the one with 

 103 tentacles. The most usual number for the tentacles is from 60 to 90 J. 

 The tentacles are always of more than one size §, and usually three distinct 

 sizes or orders are present. The proportion of those of the first (largest) 

 order to the rest varies from one-ninth to one-half — one-third || being the 

 proportion most frequently found. As Ascidia mentxda usually lies attached 

 with the left side downwards, it would not be surprising to find that some of 

 the apparent irregularities in size and position of the tentacles bore some 

 definite relation to the two sides of the body, but no such relation exists. In 

 some specimens it is the right and in others it is the left side that has -the 

 largest or the greatest number of tentacles. One individual examined had 

 4 tentacles of the first order on the right side and 3 on the left, while 

 another with 4 tentacles on the right side had no less than 11 on the left. 

 There is a considerable range of individual variation in the spacing of the 

 tentacles, which may be densely crowded (Pi. 19. fig. 7) or sparsely scattered 

 (fig. 5), or show intermediate gradations. Figures 5 and 6 show the 



* We are not at the moment concerned with the question whether or not it is convenient 

 to separate venosa in an independent genus Ascidiella. 



t Miss H. M. Duvall, B.Sc, a post-graduate worker in my laboratory, has kindly assisted 

 me in this investigation. 



X In tlie L. M. B. C. Memoir on ''Ascidia" I find that, as the result of a number of 

 specimens examined, I then gave the number characteristic of the species as being from 



70 to 100. 



§ In Alder and Hancock's 'British Tunicata,' vol. i. (1905), u-aAev Ascidia me)ittda,we 

 are told on p. 76 that the " tentacular filaments" are of " equal length," while on p. 79 they 

 are said to " vary considerably in size." The latter is the more correct statement. 



|| This agrees with what is stated in the L. M. B. (J. Memoir. 



