xiii PHYLUM CHORDATA 31 



conclusion has yet been reached as to the function which it 

 performs. 



Reproductive System. The Urochorda are hermaphrodite. 

 Ovary and testis are in all cases simple organs placed in close 

 relation with one another. In Appendicularia (Fig. 723) they are 

 situated in the aboral region of the body. In the simple Ascidians, 

 they may be either single or double, and their ducts, sometimes 

 very short, sometimes more elongated, open close together into the 

 at rial cavity. In Pyrosoma there are no gonoducts, the ovary 

 which contains only a single ovum and the testis being lodged in 

 a diverticulnm of the peribranchial cavity. In Salpa also the 

 ovary contains usually only a single ovum : ovary and testis lie in 

 close relation to the alimentary canal in the " nucleus," and their 

 short ducts open into the peribranchial cavity. In Doliolum the 

 elongated testis and oval ovary have a similar position to that 



cf 



ph 



JT ^S 



3 



-c.c 



I [',. 78]. Salpa. Diagrammatic lateral view of the ganglion and neighbouring parts, at. 

 wall of atrial cavity ; br. branchia ; br. ap. aperture of branchia ; c. c. ciliated crests of 

 branchia ; c. j. ciliated funnel ; ey. eye ; n. t/L gland (paired) that may represent neural gland ; 

 ph. wall of pharynx. (After Delage and Herouard.) 



which they occupy in Salpa, but the ovary consists of a number 

 of ova. 



Development and Metamorphosis. In the Ascidiacea Im- 

 pregnation usually takes place after the ova have passed out from 

 the atrial cavity. But in a few simple, and most if not all com- 

 pound forms, impregnation takes place in the atrium or in a 

 special outgrowth of the latter serving as a brood-sac, and the ovum 

 remains there until the tailed larval stage is attained. In certain 

 composite forms there is a coalescence of the investing layers of 

 the ovum with the wall of the atrium, forming a structure analogous 

 to the placenta of the Mammals and designated by that term. 

 Self-impregnation is usually rendered impossible by ova and sperms 



