360 TUNICATA. 



According to van Beneden and Julin, however, the chorda does 

 not reach so far forward in Claoelina as in Phallusia. 



The caiulnl section of the nerve-cord (Figs. 167 and 168, s) is a 

 tube the walls of which consist of a simple pavement-epithelium. 

 In cross-section (Fig. 169, nr, p. 363), four cells are usually found, 

 two lying laterally, one dorsally and one ventrally. 



This section extends to the posterior end of the body. The reduction of 

 the lumen of the alimentary canal in the caudal region of the embryo is 

 accompanied by the obliteration of the neurenteric canal which represented 

 the posterior continuation of the central canal of the nerve-cord. 



Kupffer observed the important fact that, in the larva of Ascidia 

 mentula, lateral bundles of tibrillae are given off' by the caudal 

 section of the spinal cord ; these we may claim as spinal nerves. 

 The first pair of these was found on the boundary between the 

 trunk and caudal regions, and the following two pairs at intervals 

 more or less corresponding to the length of a caudal muscle-cell. 



We may regard this as an indication of the segmentation of the 

 caudal region. The same significance may be attributed perhaps 

 to those cell-groups found by Lahille (about ten in number) in 

 the caudal nerve-cord of the DistapHa larva, and also occurring in 

 Appendiailaria (Nogine, Langerhans, and others). 



The ciliated pit. In connection with the central nervous system, 

 we must describe a ciliated diverticulum which opens into the dorsal 

 wall of the anterior section of the alimentary canal (branchial sac 

 or pharynx) and which has been claimed as a homologue of the 

 Hypophysis cerebri of the Vertebrates. In adult Ascidians its form 

 is more complicated. A glandular mass, the sub-neural gland 

 (glande-hypophysaire or sub-ganglionic body) can then be recognised 

 in close proximity to the brain, and an efferent duct runs forward 

 to enter the pharynx through a complicated apparatus, the dorsal 

 tubercle, in the dorsal middle line between the two ciliated bands 



i 



(sillons pericoronitis) which run upwards from the endostyle. The 

 opening of this duct has erroneously been assumed to be an olfactory 

 organ (olfactory tubercle, see Julin, Nos. 26 and 27). 



According to van Beneden and Julin (No. 7) and Seeligek 

 (No. 50), the first rudiment of this ciliated diverticulum arises quite 

 independently of the nervous system as a pit-like invagination of 

 the entodermal wall of the pharynx. At a later stage, the blind 

 end of this diverticulum is said to become closely applied laterally 

 to the sensory vesicle, always on the side which is turned away 



