352 NERVOUS SYSTEM OF TUNICATA AND CONCHIFEEA. 



Between these orifices is the single ganglion c, 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 (YEGET. PHYS., 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. 



437. In the CONCHIFERA, or animals inhabiting bivalve 

 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 (B, 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 A), situated near the front 

 of the body, either upon or at the sides of the oesophagus, and 

 connected by a commissural band that arches over it ; these 

 ganglia receive nerves from very sensitive tentacula which 



