1 88 THE ASCIDIANS. 



from a common centre of formation, which subsequently 

 undergoes a division into two portions, one of which be- 

 comes the ovary, and the other the testis. Similarly the 

 oviduct and vas deferens are derived by division of a 

 primarily single structure, which arises in continuity 

 with, and in fact as an outgrowth from, the primitive 

 sexual gland. 



In spite of their hermaphroditism, it would appear that 

 not all the Ascidians are self-fertilising, although many, if 

 not most of them, are. In some cases it is supposed that 

 in different individuals the male and female organs attain 

 maturity at different times, so that in a given individual, 

 when the ovary is ripe the testis is unripe, so that it must 

 be fertilised from another individual, in which the testis is 

 ripe, but the ovary unripe, and so on. 



Nervous System and Hypophysis. 



{Neurohypophysial System . ) 



The central nervous system of an Ascidian usually bears 

 f a ridiculously small proportion to the bulk of the organ- 

 ism. Its main constituent is a ganglion which lies im- 

 bedded in the thickness of the mantle, between the oral 

 and the atrial siphons, the two latter structures being 

 innervated by nerves proceeding from the ganglion. As 

 belonging to the central nervous system must also be 

 mentioned a solid nerve-cord which runs along the dorsal 

 border of the branchial sac from the cerebral ganglion 

 to the visceral region (Fig. 96). This was discovered by 

 van Beneden and Julin, and is derived from a persistent 

 portion of the central nervous system of the larva. 



Beneath the cerebral ganglion is a lobulated glandular 

 organ known as the subneural gland. It is provided with 



