364 LECTURE XXIV. 



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 dioecious 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 indi- 

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

 remarkable combination of male and female parts arranged for 

 reciprocal union. The closer and more remarkable analogy with 

 tiie vegetable kingdom off'ered by that peculiar modification of the ge- 

 nerative 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. 



But the diversified structures of the Invertebrate animals not only 

 teach us the most remarkable and instructive modifications and 

 correllations of individual organs and systems, but lead to an insight 

 into, and can alone furnish the demonstrations of, the most important 

 jreneralisations in 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 this vegetative con- 

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

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

 Such sacs are developed in great numbers in the body of the Poly- 

 gastrian, but each sac performs tlie same share of the digestive 

 function, irrespective of the rest. The case is very different in the 

 ruminant animal, in which each of the four stomachs has its appro- 



