VARIATION IN ASOIDIA. 285 



will result in a considerable number o£ the named and described British 

 Ascidians being shown to have no existence as distinct species. That will 

 probably be the fate of some o£ the supposed species described by Hancock, 

 and by myself and others, from single specimens in some cases preserved in 

 spirit and possibly distorted. These descriptions may have served a useful 

 purpose at a certain stage in our knowledge of the group, but the names may 

 eventually in the light of further study and comparison have to be removed 

 from the list of valid species. 



I had hoped to have included in the present study some further series of 

 common forms of Ascidians which I had collected from the ' Runa ^ for the 

 purpose ; but other work has come in the way, and they must now be left 

 over for some future occasion when I shall hope to publish the results in 

 another part of " Spolia Runiana." 



EXPLANATION OF PLATE 19. 



Fig. 1. Fimiculina quadrangular is (Pallas), alive aud expanded in a cylinder of sea-water — 



about half natural size. From a photograph. 

 Fig. 2. Diazona violacea, Sav., alive and expanded to show the arrangenaent of the ascidio- 



zooids ; reduced a little. 

 Fig. 8. Anterior end of an ascidiozooid of Diazona violacea to show the arrangement of the 



lines cf white pigment on the thorax. Enlarged. 

 Fig. 4. Forbesella tessellata (Forbes), upper surface of the test to show the plates. Enlarged. 

 Figs 5 to 8. Ascidia mentula, Miill., showing variation in tentacles. 

 Fig. 6. Sparsely scattered arrangement of tentacles. 



Fig. 6, More crowded condition found in another part of the same individual. 

 Fig. 7. Densely crowded arrangement of tentacles. 

 Fig. 8. The typical arrangement of the tentacles in A. mentida. 

 Fig. 9. Tentacles of Ascidia venosa, Miill. — Figs. 5 to 9 all enlarged about 10 times. 



