250 FUNCTIONS OF THE NERVOUS SYSTEM. 



be called the pedal ganglion. And there is sometimes another, which espe- 

 cially supplies the mantle with nerves ; and this may be called the palleal 

 ganglion. The distribution of their nerves to the different organs, would 

 alone indicate the respective functions of these ganglia; but these are placed 

 beyond doubt, by that very great variety in the disposition of these organs, 

 which is characteristic of the Mollusca. The development of the sensory 

 organs, the situation of the gills, the structure and position of the foot, the 

 conformation and uses of the mantle, are well known to differ in the most 

 obvious manner, in genera which are closely allied to each other. Hence 

 the anatomist is enabled, by the discovery of corresponding changes in the 

 nervous system, to satisfy himself of the particular functions of its different 

 centres.* 



316. It is only in the higher tribes, however, that this separation of function 

 is evident ; for in the lowest, we find the Nervous System in its least deve- 

 loped form. This is the case in the class TUNICATA; composed of animals, 

 in which the whole body is enclosed in a tunic or bag, having two orifices, 

 through one of which the water is drawn in by ciliary action, whilst through 

 the other it is expelled. This bag incloses a large chamber, the lining of 

 which is devoted to the respiratory function ; and at the bottom of it lies the 

 mass of the viscera, on which is the entrance to the stomach. A part of the 

 water which is taken into the respiratory chamber flows into this, and passes 

 through the intestinal canal ; being discharged along with that, which has 

 only served the purpose of aerating the blood. These animals have no power 

 of motion, but such as is effected by the general contraction of the respiratory 

 sac; this is effected by a single ganglion placed between its orifices, which is 

 therefore chiefly a branchial ganglion, and is the only nervous centre they 

 possess. The trunks connected with it send branches over the muscular 

 envelope of the respiratory sac, and to the sphincters which surround its 

 orifices ; whilst other branches proceed to the membrane lining the orifices, 

 and especially to the tentacula, or lips, which are situated at the oral entrance. 

 The maintenance of the regular current is effected, as just stated, by ciliary 

 action; but when any substance is being drawn in, the entrance of which 

 would be injurious, its contact with the tentacula excites a general contraction 

 of the muscular envelope, and 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 vio- 

 lently contracts, and expels the contents of the sac. These are the chief, if 

 not the only actions, which the Nervous System of these animals is destined 

 to perform ; and they are evidently of a reflex character ; bearing a close 

 correspondence with the acts of coughing and sneezing in Man, which are in 

 like manner destined to expel injurious substances from the respiratory pas- 

 sages. By the contact of such substances with the tentacula that guard the 

 oral orifice, or with the lining of the respiratory sac, or by irritation of the 

 external surface of the body, an impression is produced on the afferent fibres ; 

 which, being conveyed to the central ganglion, excites there a reflex motor 

 impulse ; and the propagation of this impulse along the afferent fibres, to the 

 muscular fibres of the contractile sac, and to the sphincters, produces the 

 movements in question. 



317. la the CONCHIFERA, or Mollusks inhabiting bivalve shells, there are 

 invariably two ganglia, having different functions. The larger of these (Fig. 

 124, c), 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), 



* See Mr. Garner on the Nervous System of the Mollusca, in the Linnaean Transactions, 

 Vol. xvn. 



