The schoolmen will scarcely have an answer for this question, except by placing themselves on the theologico-teleological stand-point which has justly fallen into disfavour amongst us, and from which the mode of production of an arrangement is supposed to be explained, if its “adaptation” to the animal can be demonstrated. From this point of view we might certainly say that a widely gaping fissure which had nothing prejudicial in it to _ Aratus Pisonii_ among the foliage of the mangrove bushes, was not suitable to the _Ocypoda_ living in sand; that in the latter, in order to prevent the penetration of the sand, the orifice of the branchial cavity must be placed at its lowest part, directed downwards, and concealed between broad surfaces fringed with protective brushes of hair. It is far from the intention of these pages to enter upon a general refutation of this theory of adaptation. Indeed there is scarcely anything essential to be added to the many admirable remarks that have been made upon this subject since the time of Spinoza. But this may be remarked, that I regard it as one of the most important services of the Darwinian theory that it has deprived those considerations of usefulness which are still undeniable in the domain of life, of their mystical supremacy. In the case before us it is sufficient to refer to the Gelasimus of the mangrove swamps, which shares the same conditions of life with various Grapsoidæ and yet does not agree with them, but with the arenicolous _Ocypoda._ [1] As this was not observed in the sea, but in glass vessels containing sea-water, it might be supposed that the animals become exhausted and die, not because they are under water but because they have consumed all the oxygen which it contained. I therefore put into the same water from which I had just taken an unconscious _Ocypoda,_ with its legs hanging loosely down, a specimen of _Lupea diacantha_ which had been reduced to the same state by being kept in the air, and this recovered in the water just as the _Ocypoda_ did in the air. [2] This smaller _Gelasimus_ is also remarkable because the chameleon-like change of colour exhibited by many Crabs occurs very strikingly in it. The carapace of a male which I have now before me shone with a dazzling white in its hinder parts five minutes since when I captured it, at present it shows a dull gray tint at the same place. CHAPTER VI. STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA. Scarcely less striking than the example of the air-breathing Crabs, is the behaviour of the heart in the great section Edriophthalma, which may advantageously be divided, after the example of Dana and Spence Bate, only into two orders, the Amphipoda and the Isopoda.