886 THE NERVOUS SYSTEM IN GENERAL, [ch. tt. 



parts of the embryo^ but from all time, doubtless, the bones, 

 muscles, and membranes had each their own material/^ 



Seeing that sensation and voluntary motion in man, and 

 many animals, are obtained through the nerv^ous system, we 

 conclude from analogy, that all animals which feel and have 

 voluntary motion, possess a nervous system ; and so essential is 

 that structure, that no animal exists without it, from man 

 down to the smallest microscopic insect. Haller, however, 

 correctly shows, that this analogy does not hold good, and 

 observes,^ that in some animals, as polypi and zoophytes, no 

 nervous system has been discovered ; and since these animals 

 manifestly belong to the animal kingdom, their difference from 

 vegetables does not consist in having nerves. Spallanzani more 

 fully illustrates the abuse of the argument from analogy .2 He 

 says that if w*e examine the whole animal chain, it will appear 

 to us at first, passing from man to quadrupeds, that each have 

 their organs of digestion, circulation, various secretions; and 

 each also their nervous system, muscles, bones, and organs of 

 the senses ; and although they diverge not a little from man as 

 to form and structure, yet as they agree in their essential use, 

 the analogy may still be allowed ; but if we gradually descend 

 to fishes, insects, and infusory microscopic animalcules, the force 

 of the analogy is very much weakened or altogether lost, for 

 we see that in insects, the bones, the heart, arteries, and veins, 

 carrying red blood, are wanting ; they have, besides, no brain, 

 although endowed with nerves, and their organs of respiration 

 resemble those of vegetables. But if we descend the animal scale 

 still lower, we find creatures in whom this entire apparatus of 

 organs is wanting, and which are entirely destitute of nerves as 

 well as of brain. This is seen in such animals as many polypes, 

 whose body is nothing more than an oblong sac made up of small 

 granules; or such as many aquatic animalcules, whose whole 

 body is simply a membrane, or vesicle ; or such as many marine 

 zoophytes, whose whole body consists only of a sort of simple 

 jelly. This astonishing simplicity of structure induced Bonnet 

 and Needham to conclude, that these animalcules are not true 

 animals endowed with an immaterial sentient principle, but 

 merely living entities endowed with irritability only. But they 



' De Part. Corp. Hum. Fabr., lib. x, sect, vi, § 1. 



' Opuscules de Physique Animal, et Vegetal., tom. i, chap. xii. 



