as supported hy Observations on Crustacea, 413 



the branchial chamber a great many times, carrying always a 

 fresh supply of oxygen with it. In moist air this circulation of 

 water may be maintained for a very long time; but when the 

 provision of water is evaporated, the Crab has recourse to the 

 posterior aperture for aerial respiration. 



The arenicolous Ocypoda* have become so completely estranged 

 from an aquatic mode of life that a stay of one day in sea-water 

 is sufficient to kill them. It has long been observed that in 

 these animals the third and fourth pairs of feet are exceedingly 

 close together. The contiguous surfaces of these legs are clothed 

 at the margins with a dense coat of hairs. It has been supposed 

 that these hairs were intended to diminish the friction of the 

 surfaces ; but this is evidently a mistake. Dr. Miiller has dis- 

 covered between the bases of these approximated legs an aper- 

 ture leading into the respiratory cavity. This arrangement 

 exists in several species of the family, in particular in certain 

 Gelasimi, some of which inhabit the mangrove-swamps, whilst 

 others run about upon the sand in open day. 



One might perhaps be tempted to give a teleological explana- 

 tion of these diflferences in the organization of the respiratory 

 apparatus, and say that the Octjpod<Ej for example, living in the 

 sand, require to have the orifice more protected against the in- 

 troduction of foreign bodies, and consequently more concealed, 

 than the Grapsidse. But this argument may be refuted by more 

 than one reason. It is sufficient to state that a Gelasimus which 

 lives far from the sands, in the mangrove-forests, in company 

 with several Grapsidse, nevertheless has the respiratory fissure 

 concealed between the third and fourth pairs of feet. 



The Crustacea present several very distinct modes of develop- 

 ment — the development of the Podophthalma, that of the Edri- 

 ophthalma, and that of the Entomostraca (including Cirripedes), 

 Certain Podophthalma issue from the egg under their definite 

 form ; this is the case in the common Crayfish {Astacus fluvia- 

 tills) and in an Indian terrestrial Gelasimus. But all the ma- 

 rine Podophthalma appear to present themselves under larval 

 forms, which is a further verification of the law which is evi- 

 dently prevalent among the Annelida, Turbellaria, and Mollusca, 

 in accordance with which the terrestrial or fluviatile species un- 

 dergo no metamorphoses, whilst the marine species are subject 

 to such changes. In any case, the Podophthalma with larvae 

 appear to be developed upon a single plan. In the larval state 

 they present the form of a Zoea. The Zoece are creatures entirely 

 destitute of a thorax, that is to say, of that region of the body, 

 which in the Crabs and Lobsters bears the five pairs of locomo- 

 tive appendages to which the Decapoda owe their name. Their 



