352 NERVOUS SYSTEM OF TUNICATA AND CONCHIFERA. 
Between these orifices is the single ganglion ¢, which sends — 
filaments to both of them, and other branches which spread — 
over the surface of the mantle d. These animals are for the — 
most part fixed to one spot during nearly the whole of their — 
existence ; and they show but little sign of life, beyond the — 
continual entrance and exit of the currents already adverted 
to. When any substance is drawn-in by the current, however, 
the entrance of which would be injurious, it excites a general — 
contraction of the mantle ; and this causes a jet of water to — 
issue from both orifices, which carries the offending body to — 
a distance. And in the same manner, if the exterior of the 
body be touched, the mantle suddenly and violently contracts. — 
436. These are the only actions, which, so far as we know, 
the nervous system of these animals is destined to perform. 
They scarcely exhibit any traces of eyes or other organs 
of special sense; and the only parts that appear peculiarly — 
sensitive, are the small tentacula which guard the orifice a. 
It would seem as if the irritation caused by the contact of any | 
hard substance with these, or with the general surface of the ~ 
animal, caused a reflex contraction of the mantle, having for 
its result the getting-rid of the source of the irritation. Such | 
a movement could only be performed by the aid of a Nervous 
system, which has the power of receiving impressions, and of © 
immediately exciting even the most distant parts of the body 
. to act in accordance with them. In the Venus’s Fly-trap and — 
Sensitive Plant (Vecrr. Puys., §§ 214, 391), an irritation © 
applied to one part is the occasion of a movement in another; _ 
but this takes place slowly, and in a manner very different | 
from the energetic and immediate contraction of the mantle | 
of the Tunicata. es 
437. In the Concurrera, or animals inhabiting bivalve 9 
shells, there are invariably at least two ganglia, having differ-— 
ent functions. The larger of these, corresponding to the single 
ganglion of the Tunicata, is situated towards the posterior end 
of the body (8, fig. 182), in the neighbourhood of the posterior 
muscle ; and its branches are distributed to that muscle, the 
mantle, the gills, and the siphons. But we find another gan- 
glion, or rather pair of ganglia (a), situated near the front 
of the body, either upon or at the sides of the cesophagus, and 
connected by a commissural band that arches over it; these 
ganglia receive nerves from very sensitive tentacula which | 
