II STRUCTURE — NERVOUS SYSTEM 53 



The neural gland, which was first noticed by Hancock, may 

 be continued backwards along with the dorsal nerve, and it 

 communicates anteriorly by means of a narrow duct with the 

 front of the branchial sac (pharynx). The opening of the duct 

 is enlarged to form a funnel-shaped cavity (Fig. 24, A), which 

 may be folded upon itself, convoluted, or even broken up into a 

 number of smaller openings (see Fig. 43, p. 79), so as to form a 

 complicated projection called the dorsal tubercle, situated in the 

 dorsal part of the prebranchial zone. The dorsal tubercle in 

 Ascidia mentula is somewhat horse-shoe shaped (Fig. 21, d.f); it 

 varies in most Ascidians (see Fig. 43) according to the genus and 

 species, and in some cases in the individual also. Sensory cells 

 are found in the epithelium, and so it is highly probable that 

 besides being the opening of the duct from the neural gland, this 

 convoluted ciliated ridge may be a sense-organ for testing the 

 quality of the water entering the branchial sac. 



Nervous System and Sense-Organs. — The single elongated 

 ganglion (Fig. 24, n.g), in the median dorsal line of the mantle, 

 between the branchial and atrial siphons, is the only nerve- 

 centre in Ascidia and most other Tunicata. It is the degenerate 

 remains of the dorsal wall of the tubular cerebro- spinal nervous 

 system of the trunk -region of the tailed larval Ascidian — the 

 ventral wall opposite having given rise to the subneural gland. 

 The more posterior or spinal part of the larva has almost entirely 

 disappeared in most adult Tunicata. It persists, however, in the 

 Appendiculariidae, and traces of it have been found in the dorsal 

 nerve running backwards towards the oesophagus in some Ascidians 

 (e.g. Clavelina). It may be ganglionated in Molgulidae. 



The ganglion has small rounded nerve-cells on its surface, 

 and interlacing nerve -fibres inside. It gives off distributory 

 nerves at both ends (Fig. 24, A), which run through the 

 mantle to the neighbourhood of the apertures, where they divide 

 up to supply the lobes and the sphincter muscles. The 

 only sense-organs are the pigment spots (" ocelli," formed of 

 modified ectoderm cells imbedded in red and yellow pigment), 

 between the branchial and atrial lobes, the tentacles at the base 

 of the branchial siphon, and probably the dorsal tubercle and 

 the languets or dorsal lamina, in all of which, as well as in 

 the endostyle and peripharyngeal bands and in papillae on the 

 ectoderm and in the branchial sac, sensory cells have been found. 



