54 ASCIDIANS 



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 Tunica ta, 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 Lies 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 cur^-e, the 

 intestinal loop, in which the o^-ary lies, 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, ty), a pad which runs along its 

 entire length, so as to reduce the lumen of the tube to a crescentic 

 slit. 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 applied, 

 as in some other Tunicata. 



Renal Organ. — A mass of large clear-walled vesicles which 

 occupies the rectal loop (Figs. 18 and 19, ren), 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 circulatino- in 

 the neighbouring blood-lacunae, and deposit them in the cavity of 



