4 INVERTEBRATA. 



may be at once sent off ; or there is a cord or prolongation, composed of two portions, 

 more or less separated, and extending from the first ganglion to a second, and so on 

 with respect to the rest. Generally the nerves arise from the ganglia, but sometimes 

 from the prolongation also. The ganglia vary in number, and are larger or smaller 

 in proportion to the quantity of nerves given off by each of them. They are more 

 concentrated when the skeleton is round or oval, when the motion of the part on 

 which they are placed is so constrained that their functions cannot be interfered with, 

 and the extremities can be more conveniently supplied by them with nerves. In the 

 crab there is one large ganglion for furnishing all the legs ; and, in the spider, there is 

 a similar disposition ; in the lobster, whose skeleton is elongated, and its different 

 parts more moveable, there are several ; but those, beyond the one giving nerves to 

 the last pair of legs, are much smaller than the rest. In the centipede, the ganglia 

 are of an almost equal size throughout, and as numerous as the legs and the divisions 

 of the skeleton ; and thus a very free motion is allowed. Creatures like the leech, 

 which are capable of great and sudden changes in the size of the body, derive the 

 highest advantage from numerous ganglia, as these allow either a contraction or 

 distension of the stomach without inconvenience ; for, whilst a simple cord in the 

 empty state might have been displaced sufficiently for interfering with the distribution 

 of its branches, in the distended state it might have been over-stretched, and the 

 functions of the whole impaired. In the slug, which is less capable of being subjected 

 to similar inconvenience, because the variations of its Alimentary canal are more 

 limited, there is one principal ganglion besides the brain, from which the nerves arise. 

 The changes that take place from the state of the larva to that of the perfect insect, 

 demonstrate the necessity for a proper adaptation of the ganglia and nerves to the 

 new form of the body. When a simple cord in the form of a long nerve does not 

 suffice, a mass ot ganglion, in larger or smaller divisions, proportionate to the requisite 

 quantity of nerves, is provided, and placed in an appropriate form for corresponding 

 with the shape as well as the locomotive necessities and other requisite changes 

 of the body. 



A great portion of the preceding observations refers to the centres and nerves 



