18 ZOOLOGY 



SECT. 



duct (Fig. 678, dct.) 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 

 forma tubercle, the dorsal tubercle, which projects into the cavity 

 of the pharynx. 



The excretory system is represented by a single mass of clear 

 vesicles, without a duct, lying in the second loop of the intestine. 

 In the interior of these are found concretions containing uric 

 acid. 



Reproductive system.- -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. Each of them contains a 

 a cavity which, like the pericardium and the cavities of the 

 nephridial vesicles, forms a part of the original coelome. Con- 

 tinuous with the cavity of each is a duct oviduct or spermiduct, 

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

 the anus. 



The development of the Ascidian is described below (p. 27). 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Urochorda are Chordata in which the notochord is confined 

 to the tail region, and, in all but the Larvacea, is found only in 

 the larva. The adults, which for the most part are retrogress! vely 

 metamorphosed, in other respects besides the abortion of the 

 notochord, are sometimes sessile, sometimes free and pelagic ; they 

 frequently form colonies (fixed or free) by a process of budding, 

 and in some instances exhibit a well-marked alternation of gene- 

 rations. The body is enclosed in a test consisting largely of 

 cellulose. The proximal part of the enteric canal (pharynx) is 

 enlarged to form a spacious sac with perforated walls acting as an 

 organ of respiration. There is a simple heart and a system of 

 sinuses, the cavities of which are remains of the blastoccele. The 

 coelome is represented, apparently, only by the pericardium and 

 by spaces in the interior of the gonads and of the renal organ. 

 The sexes are united. The larva is always free-swimming, and is 

 nearly always provided with a caudal appendage. 



Three orders of Urochorda are recognised : 



ORDER 1. LARVACEA. 



Free-swimming pelagic Tunicata with a caudal appendage, 

 supported by a skeletal axis or notochord. The test is represented 

 by a relatively large temporary envelope, the " house," formed with 

 great rapidity as a secretion from the surface of the ectoderm and 

 frequently thrown off and renewed. The pharynx has only two 

 stigmata which lead directly to the exterior. There is no atrial 



