34 THE VOYAGE OF H.M.S. CHALLENGER. 



In the Molgulidse and Cynthiidae the whole wall of the branchial sac is thrown into 

 a series of longitudinal folds running from the prsebranchial zone to the neighbourhood 

 of the oesophageal aperture. These folds project into the interior of the sac, and are 

 directed towards the dorsal lamina (fig. 6, I. -IV. hr.f.). They vary greatly in size and 

 number, and in some cases are almost (Styela grossularia, &c.) or quite (Evgyra) rudi- 

 mentary. They are especially well developed in the sub-family Cynthinse, where they 

 are often of very large size, and attain their greatest number on each side (12 or 13, in 

 Cynthia grandis, Heller). 



On these longitudinal folds the internal longitudinal bars are always more closely 

 placed than they are in the spaces between the folds, and they become more closely placed 

 the nearer they get to the crest of this fold. Hence the meshes become narrower and 

 narrower as one passes along the side of a fold from its base to the crest. As a rule, in 

 the space between two folds, the rows of meshes are all of the same size. 



dl. 



::::.- mh. 



Fig. 6.— Diagrammatic transverse section of the Branchial Sac of Styela. 

 I., II., III., IV. br.f., the branchial folds ; d.l, the dorsal lamina ; e»., the endostyle; mh., one of the meshes. 



In the Cynthiidae the stigmata are longitudinal, and are arranged in regular rows 

 separated by the transverse vessels, but in the Molgulida? some of the transverse vessels 

 usually become irregularly placed, and branch so as to disturb the regularity of the rows 

 of stigmata. Then the stigmata themselves may become very irregular, as the interstig- 

 matic vessels are usually curved so as to form more or less complete spirals. The different 

 turns of the spiral are united by irregularly placed radiating vessels, thus forming stigmata 

 of different lengths (PI. VI. fig. 2). This arrangement of spiral interstigmatic vessels 

 and curved stigmata is also found in the genera Corella, Chelyosoma, and Corynascidia 

 among the Ascidiidse, and attains its greatest development in the genus Eugyra (PL VI. 

 % 8). 



