54 ASCIDIANS CHAP. 
These, considered as sense-organs, are all in a lowly-developed 
condition. The larval Ascidians, on the other hand, have well- 
developed intra-cerebral optic and otic sense-organs (see Fig. 26, 
p. 60), and in some of the pelagic Tunicata, otocysts and pigment-~ 
spots are found in connexion with the ganglion. 
Alimentary Canal.—The mouth and pharynx (branchial sac) 
have already been described. The remainder of the alimentary 
canal is a bent tube, which in A. mentula and most other 
Ascidians les imbedded in the mantle on the left side of the 
body, and projects into the peribranchial cavity (see Figs. 18 and 
19). The oesophagus leaves the branchial sac in the dorsal 
middle line, near the posterior end of the dorsal lamina. It is a 
short curved tube which leads ventrally to the large fusiform 
thick-walled stomach, ridged internally. The intestine emerges 
from the ventral end of the stomach and soon turns anteriorly, 
then dorsally, and then posteriorly, so as to form a curve, the 
intestinal loop, in which the ovary hes, open posteriorly. The 
intestine now curves anteriorly again, and from this point runs 
nearly straight forward as the rectum, thus completing a second 
curve, the rectal loop, in which the renal vesicles lie, open 
anteriorly. The wall of the intestine is thickened internally to 
form the typhlosole (Fig. 18, ¢y), a pad which runs along its 
entire length, so as to reduce the lumen of the tube to a crescentic 
shit. The anus opens into the dorsal or cloacal part of the 
peribranchial cavity near the atrial aperture. The walls of the 
stomach are glandular, and most of the endoderm cells lining the 
tube are ciliated. A system of delicate, microscopic, branched 
tubules with dilated ends (the “refringent organ”), which 
ramifies over the outer wall of the intestine, and communicates 
with the cavity of the stomach at the pyloric end by means of a 
duct is probably a digestive gland. There is in Ascidia no 
separate large gland to which the name “liver” can be appled, 
as In some other Tunicata. 
Renal Organ.—A mass of large clear-walled vesicles which 
occupies the rectal loop (Figs. 18 and 19, ven), and may extend 
over the adjacent walls of the intestine, is a renal organ without 
a duct. Each vesicle is the modified remains of a part of the 
primitive coelom or body-cavity, and is formed of cells which 
eliminate nitrogenous waste matters from the blood circulating in 
the neighbouring blood-lacunae, and deposit them in the cavity of 
