ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 591 



The hypophysial gland is, with the nervous ganglion, situated in 

 the interoscular region, and is regarded by the author as homologous 

 with the glandular portion of the Vertebrate hypophysis. The nerve- 

 ganglion has a cylindroidal form, is elongated from before backwards 

 and flattened from above downwards. It does not increase in size 

 with the species. In all the forms that were examined, anterior and 

 posterior peripheral nerves were detected. The latter frequently 

 divide, quite close to their origin, into a number of nerve-branches, 

 but the former never give rise to more than two trunks, which go to 

 innervate the buccal region. No branch is sent to the hypophysial 

 tubercle. 



After this preliminary account, the author proceeds to a detailed 

 description of these parts in the four species Corella parallelogramma, 

 Aseidia scabra, Phallusia mentula, and P. venosa ; and after this to a 

 histological description. 



The general conclusions to which his studies have led him are 

 the following : — 



1. The orifice which, in Ascidians, serves for the entrance of 

 water into the branchial cavity is the homologue of the mouth of 

 Vertebrates. 



2. Around the mouth there is a circular zone, limited externally by 

 the pericoronal welt ; this zone constitutes the buccal region. It gives 

 insertion to the circlet of tentacles which forms the coronal circlet, 

 and it is very probably homologous to the primordial buccal cavity of 

 Amphioxus. 



3. In this welt there are two semicircular grooves. 



4. The dorsal raphe is provided with an epibranchial groove. 



5. The hypobranchial, epibranchial, and pericoronal grooves do not 

 directly communicate with one another. 



6. The two latter are invested by a vibratile epithelium, and they 

 probably have the function of driving towards the oesophagus the 

 nutrient matters which are collected into a mucous mass by the aid 

 of the secretion of the hypobranchial groove. 



7. The ganglion consists of an external layer of grey matter, 

 which is solely formed of unipolar ganglionic cells, and of an inner 

 white substance, which is formed of nerve fibrils and smaller nerve- 

 cells. There is a complete absence of neuroglia. 



8. In some species (e. g. P. mentula) a large number of unipolar " 

 ganglionic cells lie outside this brain, in the surrounding connective 

 tissue, and their prolongations are continuous with the white substance 

 of the nervous centres. 



9. In all Ascidians there is a glandular organ which, in its 

 situation and its relations, its texture, and its probable origin, justifies 

 us in regarding it as the homologue of the hypophysis cerebri of the 

 Vertebrata. It is situated immediately below the brain, and contains 

 an excretory canal, which has just the same characters as the similar 

 canal in the Vertebrata. It opens by a ciliated infundibulum into 

 the buccal region, and its orifice is situated on a special tubercle. 



We know that at a certain period in development the hypophysis 

 of the Vertebrata opens by a widened orifice into the primordial 



2 R 2 



