IN THE BRANCHIAL SAC OP SIMPLE ASCIDIANS. 331 



ferent individuals. The stigmata are, as a rule, of much the same 

 width ; and consequently we may take the number of stigmata in 

 these spaces as a measure of their extent. The commonest 

 number of stigmata observed was sixteen, but numbers down to 

 twelve were frequent, and in one case ten only were present : 

 only once were more than sixteen observed, and in that case there 

 were twenty-three ! 



The number of internal longitudinal bars on the fold varies 

 from six to nine, and is generally eight or nine. The rest of the 

 right side of the branchial sac, between the fold and the ventral 

 clear space, is divided by the internal longitudinal bars into 

 meshes of varying size, and containing each from two to eight 

 stigmata. The prevailing numbers are six, seven, and eight ; but 

 here and there one comes across smaller meshes, generally two 

 or three together, containing each three or four (usually three, 

 rarely two) stigmata. 



On the left side of the branchial sac after the dorsal clear space, 

 and therefore in a corresponding position to the fold on the right 

 side, we invariably come upon one or more (generally two or 

 three) of these narrow meshes ; and they occur several times 

 between this point and the ventral clear space, just as they do on 

 the right side. There can be little doubt, I think, that these 

 narrow meshes are the almost obliterated or rudimentary repre- 

 sentatives of the missing folds. In several new species of Styela 

 in the ' Challenger ' collection the folds in the branchial sac 

 are in even a more rudimentary condition than the single fold 

 of Styela grossularia. They are merely tracts in which the in- 

 ternal longitudinal bars are numerous and closely placed. Now 

 these narrow meshes in the present species are caused merely 

 by three or four internal longitudinal bars being placed more 

 closely together than in the rest of the area. Then the inva- 

 riable occurrence of three or four of them on the left side next the 

 dorsal clear space seems to indicate that they represent the fold 

 in the corresponding position on the right side. Finally, they form 

 generally three or four longitudinal lines down each side of the 

 branchial sac, and four (one of which is frequently very slight) is 

 the normal number of folds in the genus. 



The branchial sac of Ascidia plebeia, Alder, has a very character- 

 istic appearance, and is very constant in the size of the meshes, 

 papillae, &c. There is one point in its structure, however, which 

 seems liable to variation. As a rule, the transverse vessels are all 



LINN. JOURN. — ZOOLOGY, VOL. XV. 26 



