NERVOUS SYSTEM OF MOLLUSCA. 481 



pharyngeal sac, by the action of the cilia that line it ; and of this, a 

 part is driven into the stomach, conveying to it the necessary supply 

 of aliment in a very finely-divided state; whilst a part is destined 

 merely for the aeration of the circulating fluid, and is transmitted 

 more directly to the anal orifice, after having served that purpose. 

 These animals are for the most part fixed to one spot, during all but 

 the earliest period of their existence ; and they give but little external 

 manifestation of life, beyond the continual entrance and exit of the 

 currents already adverted to, which, being effected by ciliary action, is 

 altogether independent of the nervous system ( 234). 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 or 

 muscular envelope ; and this causes a jet of water to issue from one or 

 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, and expels the contents of the 

 sac. 



851. These are the only actions, so far as we know, which the Ner- 

 vous system of these animals is destined to perform. They do not ex- 

 hibit the least trace of eyes, or of other organs of special sense ; and 

 the only parts that appear peculiarly sensitive, are the small tentacula, 

 or feelers, that guard the oral orifice. Between the two apertures in 

 the mantle, we find a solitary ganglion, which receives branches from 

 both orifices, and sends others over the muscular sac. This, so far as 

 we know at present, constitutes the whole nervous system of the animal ; 

 and it is fully sufficient to account for the movements which have been 

 described. For the impression produced by the contact of any hard 

 substance with the tentacula, or with the general surface of the mantle, 

 being conveyed by the afferent fibres of this ganglion, will excite in it 

 a reflex motor impulse ; which, being transmitted to the muscular fibres 

 of the contractile sac, as well as to those circular bands that surround 

 the orifices and act as sphincters, will produce the movements in ques- 

 tion. 



852. In the Conchifera, or Molluscs inhabiting bivalve shells, there 

 are invariably two ganglia, having different functions. The larger of 

 these (Plate II., Fig. 1, <?), corresponding to the single ganglion of the 

 Tunicata, is situated towards the posterior end of the body (that is, the 

 end most distant from the mouth), in the neighbourhood of the posterior 

 muscle that draws the valves together ; and its branches are distributed 

 to that muscle, to the mantle, to the gills (d, d), and to the siphons (e, 

 e), by which the water is introduced and carried off. But we find 

 another ganglion, or rather pair of ganglia (a, a), situated near the 

 front of the body, either upon the oesophagus, or at its sides ; these 

 ganglia are connected with the very sensitive tentacula which guard 

 the mouth ; and they may be regarded as presenting the first approach, 

 both in position and functions, to the brain of higher animals. In the 

 Oyster, and others of the lower Conchifera, which have no foot, these 

 are the only principal ganglia : but in those having a foot, which is a 

 muscular tongue-like organ, we find an additional ganglion (b) con- 

 nected with it. This is the case in the Solen. or animal of the Razor- 



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