52 



ASCIDIANS 



All the blood-spaces and lacunae are probably derived, like 

 the cavity of the heart, from the blastocoel of the embryo, and 

 are not, like the cavity of the pericardium, a part of the coelom 

 (of endodermal origin). 



Neural Gland and Dorsal Tubercle. — In the dorsal median 

 line near the anterior end of the body, and imbedded in the 

 mantle on the ventral ^ surface of the nerve-ganglion, there lies a 

 small Glandular mass — the neural gland — which, as Julin first 



n.gl 



n.g.: --|#J '■■"■" ■■- "S" -"^ — — -mintle 



^^^^'^='=^^^^ I Peribranchial 



• cavity 



e.p.br —\ \> c^7$2l^^W^S'P ^ i-e.p.br 



Fig. 24. — Autero-dorsal part of Ascidia showing the relations of tlie layers of the body, 

 and of the nervous system. A, in sagittal section ; B, in transverse section, d.bl.s. 

 Dorsal blood-sinns ; d.l, dorsal lamina; (7. ?i, dorsal nerve; d^.t, dorsal tnbei'cle ; 

 ect, ectoderm ; e?i, endoderm ; e.p.br, epithelium of peribranchial cavity ; gl.d, duct 

 of subneural glaud ; l.v points to the ciliated epithelium covering a longitudinal 

 vessel of branchial sac; m, mantle; ?i, nerve; n.g, ganglion ; n.gl, neural gland ; 

 p.hi\ peribranchial cavity ; j^^.&, peripliaryngeal bands ; sph, branchial sphincter ; 

 t, t', test ; f.7i, tentacle. 



showed, there is some reason to regard as the homologue of the 

 hypophysis cerebri of the Vertebrate brain. Metcalf has recently 

 shown that the neural gland may be a double structure — partly 

 cerebral and partly stomodaeal — as in Vertebrates. 



The function of this gland is still somewhat mysterious. It 

 may merely form the viscid secretion which is carried along the 

 peripharyngeal bands and down the dorsal lamina. On the other 

 hand, it has been suggested that the function of the organ may 

 possibly be renal, for the removal of nitrogenous waste matters in 

 the neighbourhood of the nervous system. Finally, it may be a 

 lymph gland. 



' Except in Cynthiidae and Botryllidae where it is dorsal. 



