358 LIVERPOOL MARINE BIOLOGY COMMITTEE REPORT. 



nalis with only six lobes, in place of eight, round the 

 branchial aperture. 



The mantle very rarely gives definite characters which 

 can be made use of in specification, but, in some cases, its 

 proportions and the general appearance of its musculature 

 afi'ord indications to those who are familiar with the species. 

 The mantle is also of importance in distinguishing some 

 genera {e.g., Ciona and Molgula). 



Probably the most important organ is the branchial sac. 

 It gives characters which serve to distinguish families, 

 genera, and species. Its larger features, such as the pre- 

 sence or absence of folds, and the arrangement of the 

 internal longitudinal bars, are of great importance in classify- 

 ing the Simple and Compound Ascidians, and these charac- 

 ters are constant. 



Other less conspicuous features, such as those derived 

 from the transverse vessels, the meshes, and the stigmata, 

 are useful in distinguishing species, and should always be 

 described, but they are liable to a great deal of variation, 

 especially towards the dorsal and ventral edges of the sac. 

 (Figure 8 on Plate IX. shews an example of this in the case 

 of Ascidia pleheia.) Consequently, the part of the wall 

 selected for examination should be taken from about the 

 middle of one side of the branchial sac. 



It is not uncommon in the Ascidiidae to find that, 

 towards the edges of the sac, the internal longitudinal bars 

 become broken up and imperfect, so as to be reduced to 

 irregular split papillae, attached to the transverse vessels at 

 the corners of the meshes (PI. IX, fig. 8). I figured this 

 condition in 1880,* in the case of an Ascidia, and in 

 1882, t in Corella jai^onica-, since then I have met with it 

 in a number of other species of Simple Ascidians, and 



* Journ. Linn. Soc, ZooL, vol. xv, pi. xvi, fig. 6. 

 t " Challenger" Expedition Keport, Zool., No. xvii, pi. xxvi, fig. 8. 



