40 ASCIDIANS CHAP. 
Endostyle.—The mucus just referred to is produced in the 
long canal-shaped gland called the endostyle or hypobranchial 
groove, which runs along the entire ventral edge of the 
branchial sac (Fig. 18, end). The sides, and especially the floor 
of the endostyle, are richly ciliated, while there are four (or six) 
strongly-marked, peculiarly-shaped glandular tracts, two (or 
three) on each side (Fig. 20, g/) running along its length, and 
separated by areas of closely-packed fusiform cells with short 
cilia, amongst which are found some bipolar sensory cells. 
This organ corresponds to the hypopharyngeal groove of 
Bet, Qe 
Fic, 20.—Transverse section of the endostyle of Ascidia mentula, x 350. 6l.s, Blood- 
sinus ; end.l, lips of the endostyle ; g/, glandular tracts ; 7./, internal longitudinal 
bar ; Z.v, interstigmatic vessels ; m, mantle ; p.67, peribranchial cavity ; sg, stigmata ; 
v.bl.s, ventral blood-sinus. 
Amphioxus and the median part of the thyroid gland of Verte- 
brata. It is interesting to notice that the (at least) four longitudinal 
tracts of gland-cells are of remarkable constancy, being found 
not only in all groups of Tunicata, including even the pelagic, 
tailed Appendicularians, but also in Amphioxus and in the 
young thyroid gland of the Ammocoete. When, in Ascidians, a 
third marginal glandular tract is added it has a different appear- 
ance from the two characteristic tracts. The mucus is carried 
forward by the action of the large floor-cilia of the endostyle 
(Fig. 20) to the groove between the peripharyngeal bands, and 
after encircling the anterior end of the branchial sac and collect- 
ing the food particles, it passes backwards along the dorsal edge 
