18 



PHYLUM TUNICATA (UROCHORDA). 



their ducts, the terminal parts of which are usually single and 

 open into the atrial cavity. The ovaries and testes are generally 

 placed near one another and contained either amongst or near 

 the digestive viscera or in the mantle wall. In the Potyclinidae 

 they are in the post-abdomen. In dona (Fig. 13) the testis is 

 a branched gland ramifying on the wall of the intestine between 

 the pylorus and the rectum. Its tubules gradually collect 

 into one main duct, which accompanies the rectum and opens 

 into the atrium in front of the anus by several openings, the walls 

 of which contain a red pigment, consisting of red renal vesicles 

 of the kind referred to above. The ovary is a rounded mass 

 placed in the intestinal loop, and the oviduct accompanies the 

 vas deferens to open close by it, far forward into the atrium. 



The gonads arise from a common 

 mesodermal rudiment. In the synas- 

 cidians they do not as a rule appear 

 in the zooid which develops from 

 the egg, and they may be absent 

 from the first-formed generation of 

 budded zooids (BotryUus. etc.). 



Reproduction by budding occurs 

 in a large number of Tunicata. 

 Sometimes, as in the synascidians. 

 the budded individuals remain em- 

 bedded in a common tunic with 

 the parent ; sometimes, as in 

 Tinliacea, they become eventually separate and lead for a 

 time an independent existence. In the latter case the life 

 history is complicated by the phenomenon of alternation of 

 generations of the variety known as metagenesis ; for the indi- 

 viduals which bud proceed from the egg and do not develop 

 sexual organs. Budding does not occur in the Appcwliculnriae 

 and monascidians. In all cases excepting BotryUus the budding 

 is effected by the division of a ventrally placed process of the 

 body called the stolon. The stolon contains a diverticulum of 

 the pharynx known as the spicardium and given off between the 

 endostyle and the oesophageal opening (p 15). The stolon also con- 

 tains an extension of the mesodermal and vascular tissues of the 

 parent. For a more detailed account of the phenomenon and 

 of the origin of the or<rm< in the bud, we must refer the reader 



Fio. 13. Intestinal loop and 

 -'I'liital organs of dona int<-x- 

 tinn/ix (t'rnn Vogt and Yunsi). 

 a ovary ; & oviduct ; c vas def- 

 erens ; dtesticular tubes raini- 

 I'yinnon the yut-wall ; < >tmii- 

 ach: / intestinal limp: ij n'ctiini. 



