622 INVERTEBRATE MORPHOLOGY. 



sac whose walls are perforated by numerous slits or pores 

 termed stigmata (Fig. 283, st), arranged in transverse or spiral 

 rows. The bars separating the stigmata enclose lacunae 

 which place the ventral and dorsal branchial lacuuse in com- 

 munication so that the walls of the sac are richly supplied 

 with blood, opportunities for its aeration being provided by 

 currents of water drawn by the cilia which border each stigma 

 through the mouth and out into the atrial cavity, whence it 

 escapes by the atrial aperture. The transverse bars which 

 separate the rows of stigmata are generally stouter than the 

 longitudinal ones, and in most species there is a second series 

 of longitudinal bars lying on the inner surface of the sac, less 

 numerous than the bars which separate adjacent stigmata, 

 united with each transverse bar by a short connecting branch, 

 and bearing opposite each junction a hollow papilla which 

 projects into the cavity of the branchial sac. Running along 

 the entire ventral mid-line of the sac is a ciliated groove (Fig. 

 283, Eri) bounded on each side by a distinct longitudinal ridge. 

 This is the endostyle, comparable to that of Amphioxus, and 

 from its anterior end a band of ciliated cells (pc] passes dor- 

 sally on each side of the pharyngeal wall to unite in the dorsal 

 mid-line. In front of these bauds another pair running par- 

 allel to them is usually found, the two pairs forming the 

 peripharyngeal ciliated bauds. From the dorsal point of 

 union of the two posterior bands a ridge, the dorsal lamina, 

 extends backwards in the dorsal mid-line of the branchial sac, 

 and in man}- species (Fig. 283) is produced into a number of pro- 

 cesses succeeding one another at intervals, and projecting into 

 the branchial cavit}^ ; these are termed the dorsal languets. 



The remaining portions of the digestive tract is in the 

 simple Ascidiaus generally situated in the mantle on the left 

 side of the body, owing to the enormous development of the 

 branchial sac, but in other forms it constitutes a part of a 

 visceral mass lying immediately below the posterior end of 

 the sac. The ossophagus, beginning at the lower end of the 

 sac, forms a short tube which opens into a fusiform stomach, 

 from the further end of which the intestine (/) arises. This 

 is generally bent twice upon itself, forming thus two loops, 

 and ends in a straight piece, the rectum, which opens by the 



