II STRUCTURE RENAL AND REPRODUCTIVE ORGANS 55 



the vesicle, where they form one or more concentrically laminated 

 concretions of a yellowish or brownish colour, sometimes coated 

 with a chalky deposit. These concretions contain uric acid, and 

 in a large Ascidian are very numerous. The nitrogenous waste 

 products are thus deposited and stored up in the renal vesicles in 

 place of being excreted from the body. In other Ascidians the 

 renal organs may differ from the above in position and structure : 

 but in no case have they any excretory duct, unless the neural 

 gland is to be regarded as one of the renal organs which has not 

 yet been proved. 



Reproductive Organs. Avidia mentula is hermaphrodite, 

 and the reproductive organs lie with the alimentary canal, on the 

 left side of the body (Fig. 19, ov). The ovary is a ramified gland 

 which occupies the greater part of the intestinal loop. It con- 

 tains a cavity which, along with the cavities of the testis, is derived 

 from an embryonic coelom ; the ova are formed from its walls, 

 and fall when mature into the cavity. The oviduct is continuous 

 with the cavity of the ovary, and leads forward alongside the 

 rectum, finally opening near the anus into the peribranchial 

 cavity (Fig. 18, g.d}. The testis is composed of a great number 

 of delicate, branched tubules, which ramify over the ovary and 

 the adjacent parts of the intestinal wall. These tubules terminate 

 in ovate swellings. Near the commencement of the rectum the 

 larger tubules unite to form the vas deferens, a tube of consider- 

 able size, which runs forward alongside the rectum, and, like the 

 oviduct, terminates by opening into the peribranchial cavity close 

 to the anus. The lumen of the tubules of the testis, like the 

 cavity of the ovary, is a part of the embryonic mesoblastic space, 

 and the spermatozoa are formed from the cells lining the wall. In 

 some Ascidians (certain Molgulidae and Cynthiidae), reproductive 

 organs are present on both sides of the body, and in others, 

 as in Polycarpa, there are many complete sets of both male and 

 female systems attached to the inner surface of the mantle on 

 both sides of the body and projecting into the peribranchial 

 cavity. 



Embryology and Life-History of a Typical Ascidian. 



The eggs of Tunicata are for the most part of small size, 

 nearly colourless and transparent, and with little or no food-yolk. 



