118 
THE VOYAGE OF H.M.S. CHALLENGEE. 
and tough, and quite opaque except in thin sections or at the edges, where it has a 
slightly hyaline appearance. In its histology the test differs considerably in different 
regions. The outer layer in the head contains great numbers of bladder cells, and the 
matrix is consequently reduced to a reticulum (PL XVII. fig. 8). Between the bladder 
cells numbers of the usual small rounded and fusiform cells are present, and here and 
there large pigment corpuscles are met with. These are like bladder cells in shape, and 
are usually larger, but they are filled with granular masses of opaque white pigment 
(PL XYll. fig. 8,p.c.). 
The peduncle is traversed by longitudinally running canals which contain the vascular 
appendages. These canals are nearly circular in cross section (PL XVII. fig. 5), and give 
the tissue its coarsely spongy texture. The test between these canals is in its outer parts 
(PL XVII. fig. 6) filled with bladder cells and great numbers of small cells, many of which 
are most beautifully branched, their processes extending through the structureless matrix to 
great distances. In the centre of the peduncle the tissue is much more dense. Bladder 
cells are absent, while the smaller cells are very peculiar ; they are nearly all larger than 
elsewhere, and of a circular or rudely square or oblong shape (PL XVII. fig. 7 ,p).c.). They 
stain with picrocarmine a homogeneous red tint, and show no nuclei. 
The mantle is singularly weak (PL XVII. fig. 9). In some cases the muscle bundles 
contain only a single delicate muscle fibre each. On these the nuclei are seen as 
distinct fusiform swelliugs. The atrial siphon is large, but has no distinct sphincter 
(PL XVII. fig. 9, at.). 
The transverse vessels of the branchial sac (PL XVII. fig. 3, tr.) are rather wide, and 
have delicate horizontal membranes. The stigmata are very like those of Colella 
gaimardi (PL XIV. fig. 8.). 
The alimentary canal is narrow (PL XVII. fig. 9). The oesophagus is long but narrow; 
it runs nearly directly backwards, and opens into the anterior end of the somewhat ovate 
stomach (PL XVII. fig. 9, st.). The stomach is not large, and its walls are smooth. It 
tapers posteriorly towards the intestine, which runs backwards and turns round ventraUy 
and then anteriorly forming a narrow loop. It crosses the oesophagus to become the 
rectum, running up the dorsal edge of the branchial sac. 
The reproductive organs form a large ovate mass placed in the intestinal loop, and 
extending beyond the intestine ventrallyand posteriorly (PL XVII. figs. 9, 10,^. and t.v.). 
It was formed entirely in all the Ascidiozooids examined by opaque ovate spermatic vesicles. 
Each of these has a short narrow duct (PL XVII. fig. 11), and these join together at the 
posterior end of the body to give rise to the vas deferens (PL XVII. figs. 9, 11, v.d.). 
This tube is conspicuous throughout its whole course. It curves round ventrally and 
then anteriorly from its point of origin, and ascends along the ventral edge of the visceral 
mass until it reaches the rectum. At its upper end for a short distance it becomes 
swollen to about twice its ordinary calibre. 
