88 
THE YOYAGE OF H.M.S. CHALLENGER. 
side of the branchial sac in its lower half, and terminates by opening into the atrial 
cavity. The anus is inconspicuous, being furnished in the adult Ascidiozooid with no 
lips, lobes, or raised margins, such as are seen in many Compound Ascidians. In a 
young specimen observed, however, the rectum was provided at its termination with a 
thickened border. It is just possible that this difference may be due to the fact that in 
the latter case the rectum was empty, while in the adult Ascidiozooids it was always 
found distended with fsecal matter. 
From the above course it follows that the alimentary canal first runs dorsally, then 
ventrally, then ventrally again, and finally dorsally, and therefore describes a curve of 
which the first half has the convexity dorsal, and the second half has the convexity 
ventral. 
The wall of the intestine has much the same structure as that of the stomach. On its 
inner surface, however, an arrangement of large clear cells may be seen, placed generally 
in a single row, and forming a wide-meshed network. No distinct nuclei are visible, 
but in some granules are present. As no concretions are ever found in them they 
cannot represent the renal vesicles seen on the intestine of some Simple Ascidians. Pro- 
bably they secrete a digestive fluid and correspond to the gland which has been described 
as occupying a similar position in many other Compound Ascidians (see p. 22). 
Blood- Vascular System . — The heart lies on the right side of the abdomen, close to 
the inner (ventral) edge of the stomach. It is a delicate fusiform tube with undulating 
walls, and is enclosed in an outer also very delicate membrane, the pericardium, which 
can only be made out with difficulty. At its anterior end the heart gives off two large 
vessels which run upwards to the thorax alongside the intestine, and are soon lost to 
view. At the posterior end three vessels are given off from the heart. One runs 
upwards along the stomach, another along the intestine, and the third is continued 
backwards into the vascular appendage, which traverses the peduncle and plays an 
important part in the process of gemmation. 
Reproductive Organs . — The ovary is placed on the left side of the abdomen 
(PI. VII. fig. 7), and, in the young condition, is confined to the intestinal loop. When 
it has attained its adult size, however, it spreads far beyond, covering the intestine in its 
lower part and extending beyond it on the ventral side enclosed in a sac-like diverti- 
culum of the mantle (PI. VII. fig. 8). The ova are distinctly visible, and vary in size 
from about 0'02 mm. to 0‘15 mm. in diameter; the larger ones exhibit the germinal 
vesicle and germinal spot. The smallest ova observed are collected together to form a 
mass which lies generally at the base of the ovary (PI. VII. figs. 9, 10). These young ova 
are irregular in shape, and are often triangular from mutual pressure. They have distinct 
strongly refracting nuclei resembling the germinal spots of larger ova, and the surround- 
ing protoplasm stains very deeply with carmine. The mature ovum (PI. VII. fig. 11) 
is spherical and of a yellowish-brown colour. It has a large germinal vesicle and a 
