INVERTEBRATA. 3 



to those they supply ; without this careful arrangement, the motions of the bones and 

 muscles might interfere with their functions, they might be too bulky for easy flexion 

 and extension, or too slender for a connection with structures possessing considerable 

 power and motion. 



When the lengthened appendage to the brain, the spinal cord, ceased to be 

 necessary, analogy would have pointed out as a substitute for it one or more nerves 

 arising from a brain or ganglion, and giving off branches in its course to the 

 contiguous parts. But it would have been difficult even to have conducted safely a 

 simple cord along the moveable rings of some invertebrate animals. If parts of much 

 extent were to be furnished, it must have proceeded from a large brain or ganglion ; 

 it must have been of considerable thickness, and therefore ill-accommodated to the 

 surrounding structures ; but it could not then have been even contained in the 

 narrowed portions of the body of many insects ; or, if it had been flat and extended, 

 so as to have been fitted to the form of a sufficiently spacious external shell or 

 covering, it might have been very inconveniently placed for giving off branches to the 

 powerful muscles. Although one large ganglion is adapted to the oval form of the 

 crab, it would have been very unsuitable to the lengthened body of the lobster, which 

 requires the greatest portion of its nerves to pass in one direction. In the tail of the 

 crab, a nerve of moderate size suffices for the slender muscles ; but it would not for 

 the large and powerful ones in that of the lobster. For producing, therefore, a 

 convenient distribution of the nerves in the varying forms of many invertebrate 

 animals, ganglia of different sizes have been adopted and placed at proper distances 

 from each other. 



In invertebrate animals the most simple form or type of the nervous system 

 exists as a cord or ring giving filaments to the contiguous parts. In very numerous 

 instances there is a brain or ganglion placed above or on the dorsal portion of the 

 oesophagus, and a longer or shorter nerve passing from it on each side of this canal 

 to the suboesophageal ganglion, situated below or on the ventral parietes ; a nervous 

 collar is thus produced, and when the suboesophageal ganglion is circular, there is the 

 appearance of a double ring. From the brain and this ganglion the principal nerves 



B 2 



