REPORT ON THE TUNICATA. 35 



In the Ascidiidse and the Clavelinidae, there are no large folds in the branchial sac, 

 like those characteristic of the Cynthiidse and the Molgulidse, but in many species of the 

 genus Ascidia, the interstigrnatic vessels are inserted into the large transverse vessels in 

 an undulating, in place of a straight line (see fig. 7, and compare with fig. 5), so that the 

 stigmatic part of the branchial sac, the region between the large transverse vessels, is thrown 

 into a series of slight projections and hollows, which gives the sac a peculiar and compli- 

 cated appearance, especially from the exterior (PI. XXVIII. figs. 3 and 4, and PI. 

 XXXIII. fig. 3), where it is not obscured by the presence of the internal longitudinal bars. 

 This "minute plication" may be further complicated by the transverse vessels being of 

 different sizes, and the slighter ones being involved in the undulations, while the larger 

 ones are not. 



In most of the Ascidiidse the internal longitudinal bars are provided on their inner 

 surfaces with papillae or knobdike projections (figs. 4 and 7, p.) placed at the points of 

 intersection with the transverse vessels, and immediately opposite the connecting ducts ; 

 and the broad membranes which hang from the front of the transverse vessels (figs. 

 5 and 7, h.m.) are attached to the side of these papillae and stretch between them like 

 horizontal shelves. In some species of Ciona and Ascidia one or two apertures of 

 communication with the peribranchial cavity, in addition to the ordinary stigmata, have 

 been found. They are comparatively long slits, in the posterior dorsal region, usually 

 one on each side of the dorsal lamina near its posterior end, and are bordered by cilia 

 much finer than those of the ordinary stigmata. 



irn'igh 



Fio. 7.-Diagrammatie horizontal section through a Mesh of the Branchial Sac of Ascidia, showing the arrangement of the fine 



longitudinal vessels which causes " minute plication." 

 tr., transverse vessel ; l.v., fine longitudinal vessel ; U., internal longitudinal bar ; c.d., connecting duct ; p., papilla ; h.m., horizontal 

 membrane. " Crest " indicates the highest, and "trough" the lowest part of the undulation. 



In the Clavelinidaa, the branchial sac is neither folded nor plicated, and there are no 

 papilke on the internal surface. In the genus Clavdina the entire system of internal 

 longitudinal bars is absent. 



In the Ascidise Composite the branchial sac is always simple in structure, differing 

 little in most forms (e.g., Botryllus) from the arrangement found in Clavdina, In some 

 cases, however, internal longitudinal bars are found, and occasionally these bear slight 

 papillae (Distoma, Diazona). 



(ZOOL. CHALL. EXP. — PART XVII. 1882.) K 6 



