ASCIDIA. 51 
the thyroid gland of Vertebrata. 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 
cesophageal opening. The endostyle, in addition to its 
elandular 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. 
DoRsAL LAMINA. 
The mucus formed by the glands in the lateral walls of 
the endostyle is carried forwards by the lashing action of 
the long cilia 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 cesophagus, 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 
alterior end of the sac, and from this point are carried 
