2 INVERTEBRATA. 



chain of similar ganglia, which give off branches to the legs, the air-tubes, and other 

 structures. In the leech the brain and ganglia are rather smaller than in the 

 preceding, but the distribution of nerves is on the same plan. In the earth-worm, 

 which is still more simple, the brain is very diminutive, and the long cord has the 

 appearance of a nerve giving off lateral branches. There is another form of part of 

 the nervous system, in which there is a thick ring instead of the lengthened chain 

 of ganglia. In the crab the brain is seen giving off the optic nerves, and others to 

 the feelers and contiguous parts, it then sends off the long nerve on each side of the 

 oesophagus to give filaments to the stomach and branchiae, and pass to the thick 

 ganglionic ring ; from this branches are sent towards the mouth, but the principal 

 portion of its branches are given to the claws and tail. In the slug the nervous 

 system approaches in some degree to that of the crab ; the brain is placed above the 

 dorsal portion of the oesophagus, and gives branches to the feelers and parts about 

 the mouth : beneath the oesophagus there is a solid ganglion, instead of the ring in 

 the crab, for supplying the fleshy foot, the integuments, the heart, and the digestive 

 organs. In the whelk there is some resemblance of the nervous system to that of the 

 slug, but it is still more simple ; there is a double ring, through which the bloodvessel 

 and intestine pass, and from which nerves are sent to the proboscis, the fleshy foot, 

 and viscera. 



The nervous system requires to be so adapted to the skeleton, that a sudden 

 shock cannot easily be communicated to it, otherwise its functions would be liable 

 to be momentarily and permanently annihilated. A peculiar conformation is also 

 necessary for preventing a compression or overstretching of such parts of it as are 

 subjected to motion ; in the spinal cord this is answered by the small circumference 

 of this organ when the motion of the vertebras is considerable, and when it is still 

 greater, as in the hedge-hog, by a long cauda equina, the short and thick spinal 

 cord being placed where very little flexion is allowed. The same precaution is 

 also required respecting the nerves, and is fulfilled by their mode of origin, their 

 proper adaptation to the openings through which they are conducted, and their 

 proportionate size to the parts they are connected with in their passage, as well as 



