REPORT ON THE TUNICATA. 
85 
The branchial sac is shown by its relations and development to be a pharynx greatly 
enlarged and modified to serve as a respiratory organ. The water, when the muscular 
mantle is relaxed, enters the sac by the branchial siphon and streams through the 
stigmata into the peribranchial space, bathing in its passage all the thin-walled vessels 
and aerating their contained blood. The current of water is directed by the lashing of 
the cilia bounding the stigmata. In nearly every specimen examined one or two small 
Copepod Crustaceans were found living in the branchial cavity as commensals. 
Enclostyle . — The endostyle runs along the ventral edge of the branchial sac 
(PI. y. fig. 13 en). It begins anteriorly at the base of the branchial siphon imme- 
diately behind the peripharyngeal band and runs backwards to the base of the sac, 
ending at the ventral edge of the oesophageal aperture. The ends are bluntly conical 
(PI. VI. fig. 4), otherwise it is of the same breadth throughout. Its course is far from 
being a straight one. In its longitudinal and lateral plane (from side to side) it forms 
a series of minute undulations, which are sometimes especially marked in its anterior 
part (PI. VI. fig. 3, en.), wdiile in its longitudinal dorso-ventral plane (PI. V. fig. 13) 
it describes two large curves of nearly equal size with the convexity outwards (ventrally) 
and separated by a deep depression ; these form the B-shaped ventral edge of the 
branchial sac. 
The endostyle is a groove with greatly thickened sides formed of columnar epithelium, 
while the base is covered with squamous epithelium. The summits of the edges are 
continued up as lip-like folds, which sometimes arch over so as to form what seems to l)e 
a canal. A tract along each side and the base are richly ciliated. Seen from the dorsal or 
ventral aspects, the endostyle shows a pair of thick brown semi-opaque bands separated 
by a more translucent area (PI. VII. fig. 2). The opaque bauds are caused by the 
thickened sides, separated by the less massive fioor of the groove. 
Dorsal Lamina . — This is probably the best designation for tlie very variously named 
organ which runs along the dorsal edge of the Ijraiichial sac opposite to the endostyle. 
It varies greatly in the details of its structure in different Ascidians, but two chief 
modifications are generally recognised — ( 1 ) where it occurs as a lamina or membrane, and 
(2) as a series of tongue-like processes or languets. These two conditions of the organ 
look very different, but are really merely the extreme modifications of intermediate stages 
which are also found. Beginning with the lamina or simple broad membrane, we find 
that this may be traversed by horizontal equidistant ribs or thickenings more or less 
strongly marked. The next stage is when these ribs, extending to the free edge of the 
membrane, form thereon, when seen in profile, a series of slight tubercles. These 
marginal tubercles are found in every degree of development, from the merest points up 
to large teeth and finally long conical languets. As a rule the breadth of the membrane 
seems to be in inverse ratio to the development of the marginal tubercles. In working 
through a series of dorsal laminae, as the tubercles increase in size the membrane 
