CONCLUDING SUMMARY. 641 



Tlie relation of the physical defence of an animal by shells or 

 crusts to the low condition of the sentient system is most strikingly 

 manifested in the Invertebrate series, and could be but feebly dis- 

 cerned in the higher animals, none of which exhibit the external or 

 dermal skeleton modified to encase the limbs and form the levers and 

 fulcra of the moving powers. The defence of the crab and oyster 

 by dense and impenetrable armour as a compensation for their lack 

 of intelligence and defective powers of action, reminds one of the 

 condition of the soldier in the early and ruder states of the art 

 of war. 



If the structure and functions of the human mechanism had been 

 illustrated only by comparison with those of other Vertebrata, the 

 physiologist would have been acquainted with only one leading 

 modification of the generative system in the Animal Kingdom, 

 namely, the dicecious or bisexual. The analogy between animals and 

 plants in the modes of continuing the species is fully illustrated only 

 by the Invertebrata. Here the anatomist finds the self-sufficing 

 combination of fertilising and productive organs in the same indivi- 

 dual, as in most flowers. Other Invertebrata present the still more 

 remarkable combination of male and female parts arranged for reci- 

 procal union. The closer and more remarkable analogy with the 

 vegetable kingdom offered by that peculiar modification of the gene- 

 rative system, in which we found the oviduct and vulva exclusively 

 destined for the transmission of the moving particles of the fertilising 

 fluid, the fertilised ova escaping into the abdomen by dehiscence of 

 the ovarium, so that extra-uterine gestation was a natural and con- 

 stant phenomenon, was demonstrated by dissection of the earthworm. 

 The small transparent vermiform parasites have afforded the best 

 subjects for the demonstration of the entry of the fertilising principle 

 into the ovum, and of the first changes therein and thereby operated. 



The diversified structures of the Invertebrate animals not only 

 teach us the most remarkable modes of action, and instructive modi- 

 fications and correlations of individual organs and systems, but they 

 lead to an insight into, and can alone furnish the demonstrations of, 

 the most important generalisations iii zootomical science. 



Of that which I have termed " the law of vegetative or irrelative 

 repetition," by which is meant the multiplication of organs performing 

 the same function, and not related to each other by combination of 

 powers for the performance of a higher function, the Invertebrata 

 afford the most numerous and striking illustrations. 



Almost every organ of the body illustrates tliis vegetative con- 

 dition at its first appearance in the Animal Kingdom. A stomach 

 or assimilative sac is the most general characteristic of an animal, 



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