EEPOET OX THE TUXICATA. 
87 
behind the tentcacles. It has a remarkably undulating course (see PI. VII. fig. 2), and is 
rather broader than the peripharyngeal band. 
Dorsal Tubercle. — This organ (PI. VII. fig. 6) is placed at the anterior end of the 
dorsal edge of the branchial sac, just in front of the end of the dorsal lamina, and in 
the bay formed by the bending posteriorly of the peripharyngeal band. It lies in the 
prebranchial zone, and is therefore anterior to the peripharyngeal band. On account of 
the blood sinus immediately superficial to it, which contains in this species a mass of 
very opaque white corpuscles (PI. VI. fig. 3, n.g.), it is only in a few cases, and with 
great difficulty, that the dorsal tubercle can be made out at all. Its outline is oval or 
nearly round, and the aperture is simple (PI. VII. fig. 6). 
Nerve Ganglion. — The nerve ganglion (PI. VII. fig. 5) is in its usual position on the 
dorsal side of the branchial aperture, nearer the branchial than the atrial aperture. It 
lies in the inner part of the mantle, and is covered by a large blood sinus (PI. VI. 
fig. 3, n.g.), on account of which it is only visible from the inner aspect. It is elliptical 
or nearly circular in outline, the ends being a little pulled out where the nerve fibres run 
outwards towards the two apertures (PI. VII. fig. 5). 
The Alimentary Canal is contained partly in the thorax and partly in the abdomen. 
Excluding the branchial sac, it may be divided into three jDarts, the oesophagus, the 
stomach, and the intestine (PI. V. fig. 13). 
The oesophageal opening is placed at the posterior end of the branchial sac, nearer the 
dorsal than the ventral edge, but jaointing towards the endostyle, from which it is 
separated by a slight elevation. A short wide oesophagus (PI. VII. fig. 7) leads back- 
wards and dorsally to the cardiac end of the large stomach. The wall of the oesophagus 
is lined by columnar epithelium, and is rather thin. Seen on the inner surface, the cells 
form an irregular minute mosaic ; their nuclei are generally not visible. 
The stomach (PI. VII. fig. 7 ; and PI. V. fig. 13, st.) is a large cavity of somewhat 
oval form, the anterior or cardiac end being the largest. Its outer (dorsal) edge is some- 
times flattened or even depressed in the centre, but is usually gently convex. The 
internal (ventral) edge is always convex, and is shorter than the outer edgQ. The internal 
surface of the stomach is not raised into any folds or rugae, such as are found in some 
other Compound Ascidians. The wall is thicker than that of the oesophagus, and is lined 
by columnar cells with shorter columnar, fusiform, or irregularly shaped smaller cells 
between and below them. As in the wall of the oesophagus, the inner ends of the cells 
present a mosaic-like appearance. 
The intestine (PI. VII. fig. 7) begins at the pyloric end of the stomach, runs 
downwards and inwards for a short distance, then turns upwards and runs nearly parallel 
with the oesophagus till it reaches the level of about the middle of the stomach, then it 
curves round to the dorsal side, crossing the upper end of the oesophagus ; and finally, 
turning anteriorly, becomes the rectum, which courses along the dorsal edge of the left 
