332 ON THE BRANCHIAL SAC OF SIMPLE ASCIDIANS. 



of the same calibre ; but in several specimens which I have exa- 

 mined every fourth vessel is much wider than the intervening 

 three. 



In Ciona intestinalis the meshes vary somewhat in size in 

 different individuals, but according to no apparent method. Five 

 stigmata in a mesh seems the normal arrangement ; four and six 

 are frequently met with, larger numbers more rarely, while ten 

 is the utmost I have observed. 



One form of variation remains to be mentioned, viz. the pre- 

 sence of delicate horizontal vessels placed irregularly between 

 the transverse vessels and dividing the meshes into two parts. 

 Ascidia aspersa, O. I\ Miiller, is an example of a species in which 

 these vessels occur as an individual variation. In typical speci- 

 mens the transverse vessels are all of the same size, and the 

 meshes are square and undivided ; but in some individuals many 

 of the meshes (not all) are traversed by these delicate horizontal 

 vessels, and so divided into pairs of transversely elongated areas. 



Notwithstanding this liability to individual variation in the 

 branchial sac of at least some species, there is no doubt that the 

 organ is of primary importance in specification. It consequently 

 seems most advisable, in the description of new species, where a 

 sufficient number of specimens is not available to determine which 

 points are constant in the species, to describe the branchial sac 

 and other important organs minutely, so that subsequent inves- 

 tigators may have details of structure in sufficient number to 

 warrant them in assuming that the great majority are constant 

 characters, and that only a few, probably, are liable to variation 

 in any one individual of the species. 



