NERVOUS SYSTEM OF MOLLUSCA AND ARTICULATA. 255 



several suckers. When the animal endeavours to embrace any object firmly 

 with its arm, it brings all the suckers simultaneously to bear upon it. There 

 can be little doubt that this action is occasioned by a motor impulse, propa- 

 gated from the cephalic masses by the non-ganglionic portion of the cord, 

 which supplies all the suckers alike. On the other hand, any individual 

 sucker may be made to attach itself, by placing a substance in contact with 

 it alone ; this action is independent of the cephalic ganglia, as is evident from 

 the fact, that it will take place when the arm is severed from the body, or even 

 in a small piece of the arm, if recently separated; and it can scarcely be doubted, 

 that it is due to the reflection of the impression made upon the sucker, through 

 the small ganglion in its own neighbourhood, where it excites a motor impulse. 

 The operation of these independent centres appears, in the entire living animal, 

 to be controlled, directed, and combined, by the cephalic ganglia ; through the 

 medium of the fibrous band which passes over them, and which mixes its 

 branches with theirs. A very similar arrangement will be presently shown 

 to exist in the double nervous column of the Articulata. 



323. Upon reviewing all the anatomical facts hitherto stated, it will be per- 

 ceived that ganglionic masses, characterized by nuclei of grey matter, or of 

 something equivalent to it, seem to exist, wherever it is desirable that impres- 

 sions made upon the afferent nerves should excite motions ; and that, as we 

 rise in the scale, there is an increase in the number of centres possessing a 

 diversity of functions. We have seen that sometimes these centres are, for 

 the sake of convenient disposition, united into one mass; whilst on the other 

 hand, when the organs are multiplied, they also are repeated to a like extent ; 

 especially when it is desirable that they should be able to act independently 

 of one another, as in the case of the suckers of the Cuttle-fish. It may further 

 be remarked, that wherever the presence of special sensory organs, confined 

 to one part of the body, gives to that part a predominance over the remainder 

 (the entrance to the alimentary canal being always in this neighbourhood), we 

 find the ganglia with which they are connected possessing a special relation 

 with all the rest, which these do not possess with each other. It is obvious 

 that, where visual organs are developed, the impressions made upon these will 

 determine the movements of the animal, more than those of any other kind ; 

 and it would seem to be chiefly owing to the information they communicate, 

 that the cephalic ganglion has such an evident presiding influence over the rest, 

 even when smaller than any of them. This is, however, more the case in 

 animals whose movements are rapid, and in which, therefore, the perception 

 of distant objects is more important as in the Insect tribes. Except in the 

 Cephalopoda, the subservience of the nervous system to the nutritive functions 

 of the Mollusca is so great that it might almost be regarded as an appendage to 

 the digestive organs, destined for the selection and prehension of aliment. 

 But in the more active members of that class it derives a more elevated cha- 

 racter, from the development of organs of special sensation and of active loco- 

 motion. 



324. The animals composing the group ARTICULATA all present, in a more 

 or less evident degree, a division into segments, which have an obvious tend- 

 ency to resemble one another, as in the Radiata ; these are disposed, however, 

 not in a circle, as in the Radiata, but in a continuous line. In those in which 

 these segments differ but little (as in the Centipede, or the Caterpillar of the 

 Insect), the nervous system is a repetition of similar parts; the most anterior 

 of the ganglia, however, has an evident predominating influence over the 

 rest, for the reason just specified ; and this influence will be found, by com- 

 parison in other classes, to diminish with the loss, and to increase with the de- 

 velopment, of the faculties of special sensation, which have their seat there. 

 The locomotive powers are just as predominant in the Articulated series, as 



