xii PHYLUM CHORDATA 319 



tion, and gives off at each end nerves which pass to the various parts 

 of the body. 



Lying on the ventral side of the nerve-ganglion is a gland — the sub- 

 neural gland. A duct runs forward from it and opens into the cavity 

 of the pharynx; the termination of the duct is dilated, and this terminal 

 dilatation is folded on itself in a complicated way to form a tubercle, 

 the dorsal tubercle, which projects into the cavity of the pharynx. 



The excretory system is represented by a single nephridium, which 

 consists of a mass of clear vesicles, without a duct, lying in the second 

 loop of the intestine. 



The sexes are united. The ovary and the testis are closely united 

 together, and lie on the left-hand side of the body in the intestinal 

 loop. Continuous with the cavity of each is a duct — oviduct or spermi- 

 duct, as the case may be — which opens into the atrial cavity close to 

 the anus. 



So far we have met with no feature that could with certainty be looked 

 upon as indicating alliances with the Chordata. But, though the adult 

 Ascidian is devoid of any such features, there is in the course of its life- 

 history a larval stage in which Chordate affinities are unmistakably 

 indicated. In this stage the young Ascidian is free-swimming, and in 

 general shape bears some resemblance to a minute tadpole, consisting 

 of an oval trunk and a long, laterally compressed tail. The tail is fringed 

 with a caudal fin, which is merely a delicate outgrowth of the thin test 

 covering the whole of the surface; running through the delicate fringe 

 are a series of strise presenting somewhat the appearance of the fin-rays 

 of a fish's fin. At the anterior end are three processes, the adhesive 

 papilla. In the axis of the tail is the notochord (noto), which at this 

 stage consists of a cylindrical cord of gelatinous substance enclosed in a 

 layer of cells. Parallel with this runs, on the dorsal side, the narrow cau- 

 dal portion of the nerve-cord, and at the sides are bands, or muscular 

 fibres. In the trunk the nerve-cord is dilated, and, further forwards, 

 expands into a vesicle, the sense vesicle {sens, ves) with an otocyst (ptd) 

 and a well-developed eye (eye). The enteric canal is distinguishable 

 into pharynx, oesophagus, stomach, and intestine. The pharynx opens 

 on the exterior by the mouth : in its ventral floor the endostyle (end) 

 has become developed; its walls are pierced by stigmata, the number 

 of which varies; a ciliated sac opens into it below the trunk part of 

 the nerve-cord. The atrial cavity has become formed round the phar- 

 ynx, and opens on the exterior by a single aperture (atr). The heart 



