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. 45, 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.¢); it 
varies in most Ascidians (see Fig. 45) 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.y), 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. 
