3i8 MANUAL OF ZOOLOGY SECT. 



a short pause, and, when the contractions begin again, they have the 

 opposite direction. Thus the direction of the current of blood through 

 the heart is reversed at regular intervals. 



The nervous system is of an extremely simple character. There is a 

 single nerve-ganglion (Fig, 186, tie. gn. ), which lies between the oral 

 and atrial apertures, embedded in the mantle. This is elongated 

 in the dorso-ventral direction, 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 connected 

 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 sperm- 

 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 such features, there is in the course 

 of its life-history a larval stage in which Chordate affinities are unmistak- 

 ably indicated. In this stage the young Ascidian (Fig. 187) is free-swim- 

 ming, 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 this 

 delicate fringe are a series of striae, presenting somewhat the appearance 

 of the fin-rays of a Fish's fin. At the anterior end are three processes, 

 the adhesive papillce. 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 caudal portion of the nerve-cord, and at the sides are bands 

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



