50 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
minute cilia which causes the regular current of water to 
flow in at the branchial aperture and out at the atrial. 
The branchial sac is very large, much the largest organ 
of the body (it may be 15 cm. in length), and extends 
almost to the posterior end of the body, while the rest 
of the alimentary canal lies upon its left side imbedded in 
the body-wall. The food particles, consisting of very 
minute plants and animals, are carried in through the 
branchial aperture by the current of water, but most of 
them do not pass out into the atrium, being caught by 
the ciliary action of the peripharyngeal bands, and entangled 
in the viscid substance which fills the groove between 
them near the anterior end of the branchial sac. This 
viscid substance, or mucus, is formed in. a long canal- 
shaped gland called the Endostyle or hypobranchial groove 
(Pl. IL., figs. 2 and 4, end.), which hes along the ventral 
edge of the branchial sac, and terminates both anteriorly 
and posteriorly in a short cul-de-sac. 
THE ENDOSTYLE. 
On the floor of the endostylar groove is found a tract 
of cells with very long cilia (Pl. IL., fig. 2). On each side are 
at least two (in some Ascidians three) laterally placed 
clumps of gland cells (Pl. IV., fig. 5), each clump separated 
from its neighbours by an area of closely packed fusiform 
cells with short cilia, amongst which are found some 
sensory bipolar cells. The lips of the endostyle are formed 
of ciliated cubical epithelium. 
This organ, on account of its thick glandular walls, 
has an opaque appearance, and seems, in side view, to 
run like a conspicuous solid rod in the more transparent 
walls of the branchial sac—hence its name endostyle. It 
is, however, really a gland, and corresponds to the hypo- 
pharyngeal groove of Amphioxus and the median part of 
