570 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
9. The Salpa, then, may be considered as a hollow cylinder, consisting of two 
tunics, an external and an internal {a, j3), the former (a) forming the mantle, the 
latter ((3) the wall of the respiratory cavity. These tunics are continuous with one 
another at the respiratory apertures, but elsewhere they are separated by a more or 
less wide space. 
In very young Salpce this space is like the cavity of a serous sac, but in the 
older forms it becomes broken up into smaller channels by the adhesion of the inner 
and outer tunics to one another at various places, and so constitutes a system of 
sinuses ; it may be conveniently called the sinus system.” 
10. Running obliquely from behind forwards and downwards, a thickish column 
or band (e) crosses the respiratory cavity ; it is hollow, and its cavity opens above 
and below into the sinus system. This is the “gill.” 
It presents an edge anteriorly and superiorly, and on each side of this, the lateral 
surfaces are beset with a series of small, oval, ciliated spaces. In this species the gill 
has but a single grand sinus running through it, and presents no appearance of vas- 
cular ramifications. The name gill has been applied to this structure somewhat too 
exclusively, as there can be little doubt that the whole respiratory cavity performs the 
branchial function. It is proposed, therefore, to call it the hypopharyngeal band, on 
the supposition that the proper respiratory cavity of the Ascidians answers to an 
enlarged pharynx. 
11. The muscular bands {k) are closely adherent to the inner tunic ; they are com- 
posed of flattened fibrils, about r^^th of an inch in diameter, which are very distinctly 
transversely striated, the striae being about ? q^q Q - th of an inch apart. The bands ap- 
pear to possess no sarcolemma. 
12. The intestinal canal (Plate XV. figs. 5 and 6) commences by a wide somewhat 
quadrangular mouth (r) opening into a flattened oesophagus, and placed at the re- 
entering angle formed by the hypopharyngeal band and the upper wall of the respira- 
tory cavity. The intestine passes backwards, then becomes suddenly bent upwards 
upon itself, and curving slightly to the right, terminates in a wide flattened anus, close 
above and to the right side of the mouth (s). 
A wide caecal sac (^), given off on the left side of the intestine and bending upwards 
and to the right side, constitutes the stomach. 
13. There is a very peculiar appendage to the intestinal canal, hitherto, it is be- 
lieved, quite undescribed, and consisting of a system of delicate, transparent, colour- 
less tubes, with clear contents, arising by a single stem from the upper part of the 
stomachal csecum, and thence ramifying over the surface of the intestine (5, 6, u), on 
what may be called the rectum, that is, the terminal portion of the intestine; it forms 
a sort of expansion of parallel anastomosing vessels, which all terminate at the same 
distance from the anus anteriorly, and from the bend of the intestine posteriorly, 
either by uniting with one another or in small pyriform cseca, Plate XV. figs. 5 and 6, 
Do these represent a hepatic organ, or are they not more probably a sort of rudi- 
