MUSEUM OF COMPARATIVE ZOOLOGY. 293 



is separated from the outer tunic ; at an early stage of development, 

 however, the outer and branchial tunics unite at the anterior extrem- 

 ity, and the central portion of the area thus united disappears by 

 absorption, so that an opening is formed through which the cavity of 

 the branchial sac communicates with the outer water ; this opening is 

 the branchial aperture. Since the test, the outer tunic, and the 

 branchial sac are all united around its circumference, the free edges 

 are composed of all three of these. Upon the inner surface of the 

 outer tunic around the aperture there is a set of muscles, by the con- 

 traction of which the opening may be entirely closed. The interior 

 of the branchial sac bears several structures which are very constant 

 in form and position throughout the group. Upon the hsemal side 

 there are two long parallel folds which project towards the ventral 

 axis of the cavity, and form the boundaries of a deep longitudinal 

 furrow (Figs. 1, 3, and 24, m), which projects as a vertical ridge into 

 that portion of the body cavity which forms the haomal sinus, but 

 remains in free communication with the branchial cavity by a cleft 

 upon its neural side. In consequence of the thickness and opacity 

 of the epithelium which lines the fundus of this fold, it appears (es- 

 pecially in the transparent Tunicata in a fresh state) like a strong 

 hollow rod, mounted upon a thin ridge-like plate, and has been called 

 the " endostyle." 



The bottom of the furrow is richly supplied with very long cilia, 

 and its sides are glandular, and secrete an adhesive slime, which 

 serves to entangle the particles of food which are carried with the 

 respired water into the branchial cavity. Upon each side of the 

 endostyle and parallel to it, there is a prominent line of cilia (Fig. 

 24, /, also 1, 3, 32 and 33, I) to which the name " epipharyngeal ridge " 

 has been given ; these two ridges are continued backward beyond the 

 posterior termination of the endostyle, where they unite to form the 

 "posterior epipharyngeal ridge," which passes backward along the 

 middle line of the posterior wall of the branchial sac to the mouth 

 (Fig. 1, o), before reaching which they again separate in Salpa, and 

 pass, one on each side of the tongue-shaped organ shown in Fig. 1, o. 

 In front of the anterior end of the endostyle the two epipharyn- 

 geal ridges diverge from each other so as to pass around the branchial 

 sac, near its anterior end, and thus form the " peripharyngeal ridges " 

 (Figs. 2, I; and 33, h). Upon the neural median line of the branchial 



