REPRODUCTIVE SYSTEM. 



407 



activity may make diffusion easier. It is interesting to find 

 such a plant-like method of storing up, instead of eliminat- 

 ing, waste products in these very passive animals. It has 

 been suggested that the sub-neural gland may have some 

 renal function. 



Heproducfive System. 



Tunicates are hermaphrodite. The reproductive organs 

 (Fig. 128, G.) are very simple, and lie in the loop of the 

 intestine. The ovary is the larger, and contains a cavity 

 into which the ova are set free, and from which they pass 

 outwards along an oviduct which opens into the cloacal 



chamber. The testis sur- 

 rounds the ovary, and is 

 mature at a different time 

 (dichogamy) ; its duct runs by 

 the side of the oviduct. In 

 some forms, where the gonads 

 are near the cloaca, there are 

 no ducts. The ova are sur- 

 rounded by follicular cells, 

 and are probably fertilised in 

 the cloaca. 



Fig. 130. — Young Embryo of 

 Ascidian (Clavellina). (After 

 Van Beneden and Julin.) 



fip., Neuropore 

 c/i., notochord ; 

 endoderm. 



neural canal ; 

 ;ctoderm ; t'«., 



Development. — Most of the As- 



cidians exhibit the development with 



metaiTiorphosis which is about to be 



described ; a few in whicli the larvte 



are retained for a long time within 



the body of the mother, show a much 



abbreviated life history. 



The fertilised ovum divides completely and almost equally. The 



spherical blastosphere becomes slightly flattened, and ultimately forms a 



two-layered gastrula. 



Along the dorsal median line of the gastrula, the ectoderm cell^ form 

 the medullary groove, the sides of which arch together and form a 

 canal — the medullary canal. This opens anteriorly to the exterior by 

 the neuropore, and posteriorly communicates with the archenteron by 

 the neurenteric canal. In the posterior region of the gut, at the sides 

 of the blastopore, a pair of diverticula, according to Van Beneden and 

 Julin, grow out. These form the mesoderm ; the endoderm cells 

 between them, roofing the gut, form the rudiment of the notochord. 

 The mesoderm masses and the notochord grow forward together for a 

 time, but later the mesoderm advances much further into the anterior 

 region, the notochord being limited to the tail. The iliverticula 

 originally contain each a small cavity — the true coslome, but this is soon 



