15 



the thyroid gland of Veitebrata. It is interesting to 

 notice that the four (at least) longitudinal tracts of gland 

 cells are of remarkable constancy, being found not only in 

 all groups of Tunicata (often six tracts) including even the 

 pelagic, tailed, Appendicularians, but also in Amphioxus 

 and in the young thyroid gland of the Ammocoete. 



The posterior cul-de-sac of the endostyle is quite short 

 in Ascidia, but in some other Tunicata it is longer and 

 becomes of great importance as an organ for the produc- 

 tion of buds. Behind the cul-de-sac the marginal folds of 

 the endostyle unite to form a slight ridge, the posterior 

 fold, which runs round the posterior end of the branchial 

 sac to join the end of the dorsal lamina behind the 

 oesophageal opening. The endostyle, in addition to its 

 glandular function, shares in the sensory functions of the 

 peripharyngeal band, the tentacles, and the dorsal tubercle, 

 in all of which similar sensory cells and nerve endings have 

 been found. 



DoESAL Lamina. 



The mucus formed by the glands in the lateral walls of 

 the endostyle is carried forwards by the lashing action of 

 the long ciHa placed on the floor of the organ, and so 

 reaches the front of the branchial sac ; here it changes 

 its direction of flow, and bends round to the right and left, 

 in the groove between the peripharyngeal bands, so as to 

 gain the dorsal edge of the sac, where it encounters a 

 projecting membranous fold, the dorsal lamina or epi- 

 branchial ridge, along which it is carried backwards — still 

 by ciliary action — to the opening of the oesophagus, the 

 next region of the alimentary canal after the pharynx. 

 The food particles become entangled in this train of mucus 

 chiefly in its course round the right and left sides of the 

 anterior end of the sac, and from this point are carried 



