136 LILIAN SHELDON. 



the exterior, and was subsequently involved in the stomodaeum, 

 or whether its opening into the mouth is primitive. In 

 Amarsecium, at any rate, it is almost certainly epiblastic in 

 origin, as it is derived from the epithelium of the stomodseum 

 and not from the pharynx, as has been stated by Seeliger 1 for 

 Clavellina. 



Since, as mentioned above, it has in the late embryo no con- 

 nection with any other structure than the brain, any other 

 connection which exists in the adult is probably secondary. 



In the adult no trace of the posterior part of the brain is found, 

 but occupying its place in Amarsecium is a mass of degenerated 

 tissue, which is connected with the exterior by means of the 

 ciliated pit. 



In Ascidia and Ciona, and apparently most other simple 

 Ascidians (cf. Julin, loc. cit.), the function of the ciliated pit 

 is to act as a duct for the so-called hypophysial gland (Julin) 

 which lies in the position occupied in the Amarsecium embryo 

 by the posterior part of the brain. 



In Clavellina the ciliated pit has a twofold function. 



(1) It communicates with the brain, and probably 

 aerates it. 



(2) Its posterior part acts as a reservoir to carry off the 

 secretion of the gland. 



There is thus a gradual transition from one function to 

 another in the different types ; the primitive condition, as an 

 organ for the aeration of the brain, is found in the Amarsecium 

 embryo, and is retained in Clavellina, while in the latter the 

 secondary function, viz. that of an excretory duct, is also 

 acquired. 



In most adult forms (e.g. Amarsecium, Ascidia, and Ciona) 

 the primitive function is lost, the secondary one only being 

 retained. 



The gland is possibly an altogether secondary structure, 

 being developed to supply the need of an excretory organ in 

 the anterior part of the body. It reaches its highest degree of 

 complication in Phallusia mammillata. 

 1 Seeliger, 0., loc. cit. 



