FORMS OF DEEP-SEA ASCIDIANS. 291 



appearing to be continuous throughout their length. These circular muscles lie beneath 

 the radial ones ; and were the respiratory cavity elongated into a cylinder, the radial or 

 longitudinal muscles would thus be internal in position, the circular external. In order 

 to prevent confusion, and because of the difficulty of drawing them clearly, not nearly 

 the entire number of transverse and circular strands is inserted in the figures in the 

 accompanying Plate (figs. 7 & 8). Their arrangement is shown in detail in fig. 12. 



Over the muscular meshwork thus formed, and extending from it to be continuous 

 in all directions with its thickened central portion, the horizontal membrane is continued 

 as a thin and transparent lamina. Opposite the indentations in the margin of the 

 thickened central portion of the membrane, i. e. between the processes or thickened 

 folds attached to the muscular bands, this thin lamina is loose and hangs in bags or 

 depressions. 



In the bottom of each of these depressions is a slight three-cornered elevation (fig. 8, a), 

 at the tip of which a perforation or aperture may possibly be present; but in the 

 specimen examined such could not be made out. At the tip of each small elevation were 

 seen only three minute dosely apposed villous folds of the membrane, between which no 

 aperture could be detected. The central horizontal opaque white membrane is merely 

 a thickened part of the general membrane, which is spread over the muscular meshwork, 

 and reaches to the tips of the large conical processes. It is probably respiratory in 

 function, and represents a gill. 



The membrane was observed to be attached to the inner surface of the test-wall at the 

 intervals between the conical processes ; but the specimen was too much injured to allow 

 of the investigation of the extent and manner of its attachment within the conical pro- 

 cesses. It appeared to be attached laterally on either side to the inner walls of these 

 processes, and is probably reflected so as to line their cavities. No normal perforated 

 gill could be discovered in any part of this membrane ; but probably the central 

 thickened portion has a respiratory function, and possibly the eight small elevations may 

 prove to be openings. No reflection of the membrane over the inner surfaces of the upper 

 and lower walls of the test was observed. 



The membrane was composed of an irregular mesh of fibrous tissue, with numerous 

 rounded gland-like cells, and numerous nerves proceeding to the radial muscular bands. 

 The mouth lies in the anterior part of the thickened portion of the membrane above 

 described, and at a short distance behind the inhalant aperture. Behind it is the endo- 

 style. The mouth is a simple oval aperture, encircled by a few sphincter muscular threads. 

 It leads directly into the digestive tract, which is embedded in a compact nucleiform mass, 

 which is seen conspicuously through the transparent test on viewing the animal from 

 beneath. The nucleus lies entirely beneath the horizontal membrane, which is attached 

 round the margin of the mouth. The exact arrangement of the viscera inside the nucleus 

 was not determined. A short tubular rectum projects from the anterior and inferior 

 extremity of the nucleus. 



Posteriorly to this, on the inferior surface, is a well-developed ovary, and behind this, 

 again, the testis. The ovary is a racemose gland, whilst the testis is composed of short 

 csecal tubes. In minute structure these organs agree closely with the testes and ovaries 

 of other Ascidians. 



