86 
THE VOYAGE OF H.M.S. CHALLENGER. 
between them becomes more and more crenated till finally a point is reached where 
the dorsal lamina exists no longer as a membrane but is replaced by a series of triangular 
or conical processes. 
In the species under consideration the dorsal lamina is in an intermediate condition. 
A membranous band extends along the dorsal edge of the sac, from the peripharyngeal 
band to the oesophageal aperture, and from this a series of long thin pointed processes 
spring at nearly equal distances. Each process or languet is placed at the intersection 
with a transverse vessel, is about 0‘1 mm. in length, is roughly triangular in outline 
(PL VII. fig. 3), and has somewhat undulating edges. A delicate membrane extends 
between the languets in a series of festoons, but does not reach quite to their points. 
Tentacles . — The tentacles are placed in a circle round the base of the branchial 
siphon just at the entrance of the branchial sac. The branchial siphon is a short funnel 
formed by the haantle and having the thickened six-lobed margin already described. Its 
inner surface is lined by an invagination of the outer test, which is of extreme delicacy 
and transparency, and extends as far down as the tentacular circlet. This point there- 
fore may really be considered as the mouth of the animal, the siphon being merely a 
depressed portion of the surface leading to the true oral aperture. 
The tentacles (PI. VII. figs. 2, 6) are simple, long, and rather stout, being large in 
proportion to the size of the animal. When directed upwards they project beyond the 
external opening of the branchial siphon. They are about sixteen in number, and are 
all nearly of the same length. They are attached by their bases to a strong muscular 
band which encircles the base of the branchial siphon, it has the lower end of the 
invaginated test attached to its upper edge, and is in relation with the prebranchial 
zone by its lower edge. 
Each tentacle is attached separately and has a swollen base, a round tapering and 
generally curved stem, and a rather blunt apex. Along one side a strong band of cubical 
ciliated epithelium is placed (PI. VII. fig. 4). The rest of the wall of the tentacle is 
formed externally of squamous epithelium (PL VII. fig. 4). In the interior, near the 
■concave side, a dark line is seen running from the base nearly to the extremity. This is 
a septum formed of connective tissue and dividing the interior into two cavities just as 
in the tentacles of Simple Ascidians. 
The Peripharyngeal Band is composed of a row of ciliated cells and encircles the 
top of the branchial sac (PL VI. fig. 3, 'p.p.). It is connected at its ventral and dorsal 
ends with the anterior extremities of the endostyle and dorsal lamina. In the bay 
formed where it curves posteriorly to meet the latter, the dorsal tubercle is placed 
(PL VII. fig. 6). 
Anterior to the peripharyngeal band is a clear space extending nearly to the base of 
the tentacular circlet. This is the zona prebranchialis (PL VII. fig. 2). Its upper 
boundary is formed by a ciliated circle like the peripharyngeal band, and placed just 
