162 
THE VOYAGE OF H.M.S. CHALLENGER. 
edge, leaving a wide prebranchial zone between its anterior end and the bases of the 
tentacles. 
The oesophagus commences at the posterior end of the dorsal edge of the branchial 
sac and runs directly backwards (PI. XXTI. fig. 4). It is a long and narrow tube with 
moderately thick walls slightly corrugated in parts. The stomach is a large ovate organ 
with its long axis placed antero -posteriorly, and with the narrower end forwards. The 
walls, which are only moderately thick, are folded longitudinally (PI. XXII. fig. 4). 
The intestine, after leaving the posterior end of the stomach, from which it is 
separated by a marked constriction, runs for a short distance backwards, and then 
turns round dorsally and anteriorly to continue its course forwards, as the rectum, 
lying at first alongside the stomach and oesophagus, and then running up the dorsal 
edge of the branchial sac to the anus, which is placed far forwards in the peribranchial 
chamber close to the atrial aperture. The intestine where it leaves the stomach is 
rather thick-walled and narrow, not much wider than the oesophagus, but while 
curving round at the posterior end it becomes rapidly wider and its wall is greatly 
reduced in thickness. The rectum is nearly as wide as the stomach (PI. XXII. 
fig. 4), and its waU is so thin as to be almost transparent. The faeces are of a 
brownish colour, and are composed mainly of Diatoms. 
The -reproductive organs forming the post-abdomen vary greatly in size, but in nearly 
all the Ascidiozooids examined they consist merely of spermatic vesicles and their ducts. 
Large ova, of an opaque orange colour, and embryos in various stages of development are 
common, however, in the peribranchial cavity, which is in some cases greatly distended 
■with them. Possibly the ova are only produced at a certain season of the year, but I 
think it much more probable that they are formed at a certain period in the life of the 
Ascidiozooid. Ova in various stages were found in one or two of the younger and 
smaller Ascidiozooids, but certainly all the large specimens which I examined were 
male. Probably the ova are formed first, and pass up into the peribranchial cavity when 
the testes begin to develop. 
Figure 14 in Plate XXII. represents a diagrammatic transverse section through 
the post-abdomen of a young Ascidiozooid. It is formed of an outer coating of 
ectoderm (ec.), inside which is a Dyer of connective tissue with muscle bands (m.h.). 
This is the mantle, and it encloses a mass of mesoderm tissue traversed by branching 
tubes (g.t.) containing germinal cells. Around these reproductive tubes are blood- 
sinuses (h.s) containing blood-corpuscles. Figures 15 and 16 show a small part of a 
section of a young post-abdomen where the germinal cells are seen in various stages of 
development into ova. Figure 17 gives a similar view of part of the post-abdomen of a 
larger and older Ascidiozooid, where the reproductive tubes have their branches much 
dilated at the ends to form spherical or ovate bulbs more or less filled with developing 
spermatozoa. The mantle and the blood-sinuses remain unchanged. Evidently the young 
