NERVOUS SYSTEM OF MOLLUSCA. 483 



the ganglia which peculiarly appertain to the suckers, and which are 

 connected with them by distinct filaments ; whilst in the other there is 

 nothing but fibrous structure, forming a direct communication between 

 these and the cephalic ganglia ; so that each sucker has a separate 

 relation with a ganglion of its own, whilst all are alike connected with 

 the cephalic ganglia, and are placed under their control. We see the 

 results of this arrangement, in the modes in which the contractile power' 

 of the suckers may be called into operation. When the animal embraces 

 any substance with its arm (being directed to this action by its sight or 

 other sensation) it can bring all the suckers simultaneously to bear upon 

 it ; evidently by a voluntary or instinctive impulse transmitted along 

 the connecting cords, that proceed from the cephalic ganglia to the 

 ganglia of the suckers. On the other hand, any individual sucker may 

 be made to contract and attach itself, by placing a substance in con- 

 tact with it alone ; and this action will take place equally well, when 

 the arm is separated from the body, or even in a small piece of the 

 arm .when recently severed from the rest, thus proving that, when it 

 is directly excited by an impression made upon itself, it is a reflex act, 

 quite independent of the cephalic ganglia, not involving sensation, and 

 taking place through the medium of its own ganglion alone. 



855. In the Molluscous classes, generally speaking, the Nervous 

 system bears but a small proportion to the whole mass of the body ; 

 and the part of it which ministers to the general movements of the 

 fabric, is often small in proportion to those which serve some special 

 purpose, such as the actions of respiration. This is what we should 

 expect from the general inertness of their character, and from the small 

 amount of muscular structure which they possess. On the other hand, 

 in the Articulated classes, in which the locomotive apparatus is highly 

 developed, and its actions of the most energetic kind, we find the Ner- 

 vous system almost entirely subservient to this function. In its usual 

 form, it consists of a chain of ganglia, connected by a double cord ; 

 commencing in the head, and passing backwards through the body (Plate 

 II., Fig. 2). The ganglia, though they usually appear single, are 

 really double ; being composed of two equal halves, sometimes closely 

 united on the median line, but occasionally remaining separate, like the 

 cephalic ganglia of the Solen (Fig. 1, #, a\ and being united together 

 by a transverse commissural trunk. In like manner, the longitudinal 

 cord, though really double (as seen in the upper part of Fig. 2), often 

 appears to be single, in consequence of the close approximation of its 

 lateral halves (as in the lower part of Fig. 2). In general we find a 

 ganglion in each segment ; giving off nerves to the muscles of the legs, 

 as in Insects, Centipedes, &c. ; or to the muscles that move the rings of 

 the body, where no extremities are developed, as in the leech, worm, &c. 

 In the lower Vermiform (or worm-like) tribes, especially in the marine 

 species, the number of segments is frequently very great, amounting 

 even to several hundreds ; and the number of ganglia follows the same 

 proportion. Whatever be their degree of multiplication, they seem but 

 repetitions of one another ; the functions of each segment being the 

 same with those of the rest. The cephalic ganglia, however, are always 

 larger and more important ; they are connected with the organs of 



