17 



so that the stigmata form more or less complete spirals. 

 In some species of Ascidia, and other Ascidians, the inter- 

 stigmatic vessels are inserted into the transverse ones in un- 

 dulating in place of straight lines, the result being that the 

 stigmatic part of the wall of the branchial sac seems to be 

 folded or thrown into microscopic crests and troughs. 

 This device for increasing the surface is known as 

 " minute plication," and is seen well in Ascidia mentula 

 (PI. IV., figs. 1 and 2). In some cases, again (Cynthiidoe), 

 the whole wall of the sac is pushed inwards at intervals 

 to form large folds visible to the eye. 



The intersections of the internal longitudinal bars with 

 the transverse vessels divide up the inner surface of the 

 branchial sac into rectangular areas called "meshes" (PI. 

 IV., fig. 2). One such mesh, containing eight stigmata in 

 a row, is seen in fig. 3, PI. II. The internal longitudinal 

 bars bear hollow papillae at the angles of the meshes, and 

 occasionally in intermediate positions. There are frequently 

 horizontal membranes (fig. 3, B. h.vi.) attached to the 

 transverse vessels between the papillae, and the transverse 

 vessels may be of two or more sizes arranged symmetri- 

 cally. There are many " connectives " running from the 

 outer wall of the branchial sac to the body -wall, and 

 allowing the blood in tlie transverse vessels to communi- 

 cate with that in the sinuses outside (PI. II., fig. 2, con.). 



In an adult medium-sized Ascidia mentula there are 

 about 150 transverse vessels on each side of the branchial 

 sac, and at least 80 internal longitudinal bars, making by 

 their intersection 12,000 meshes. The average number 

 of stigmata in a mesh is, in this species, six to eight. So 

 there may be as many as 96,000 stigmata present on each 

 side of the sac, nearly 200,000 in all. Probably these 

 numbers are greatly exceeded in large specimens, such as 

 that bliown on PI. I. These stigniata are to be regarded as 



