:io ZOOLOGY SECT. 



with ciliated funnel are present, in Doliolum ; but in Salpa, though 

 there are a pair of glands which occupy a position similar to that 

 occupied by the neural gland (Fig. 740, n. gl.), their correspondence 

 with the latter is not established, and their ducts have no connection 

 with the ciliated funnel. 



In the simple Ascidian we have seen that the renal organ 

 consists of a number of large clear vesicles situated in the loop of 

 the intestine and devoid of duct. In some forms the terminal 

 portion of the sperm-duct has glandular walls in which concretions 

 of uric acid have been found. The neural gland is by some 

 zoologists looked upon as having an excretory function, but there 

 i& no positive evidence in favour of this view, and no definite 

 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. 732) 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 

 atrial cavity. In Pyrosoma there are no gonoducts, the ovary 

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

 a diverticulum 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 

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

 of ova. 



Development and Metamorphosis. In the Ascidiacea 

 impregnation usually takes place after the ova have passed out 

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

 compound 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 Mammals and designated by that term. Self- 

 impregnation is usually rendered impossible by ova and sperms 

 becoming mature at different times ; but sometimes both ripen 

 simultaneously, and self-impregnation is then possible. 



A somewhat complicated series of membranes invests the ovum. 

 The immature ovarian ovum is enclosed in a layer of flat cells 

 the primitive follicle-cells derived from indifferent cells of the 

 ovary. On the surface of this is developed a structureless basal 

 membrane. The follicle- cells increase by division and soon form 

 a sphere of cubical cells. Certain of the cells migrate into the 



