CRUSTACEA. 183 



ration proceed ; and since the muscular energy directly depends upon 

 the amount of respiration, the two functions are brought into direct 

 relation with each other by this simple connection of their respective 

 instruments. 



The land crabs have their branchiae always supported by water 

 through special modifications of the apertures of the branchial cavities, 

 which enable them the better to retain fluid, and also by numerous 

 folds or by a spongy structure of the lining membrane of the respira- 

 tory cavity by which the quantity of the contained fluid may be aug- 

 mented. The moisture contained in the branchial chambers of the 

 land-crabs ( Gecarcinus) and tree-crabs (^Birgus) is doubtless much 

 more highly aerated than the water which bathes the branchiae of 

 the strictly aquatic species, and thus may explain the fact that the 

 Crustacea -which habitually live out of water are drowned by 

 being long immersed in that fluid. 



No other trace of distinct excretory organs has hitherto been 

 detected in the present class than the simple, unbranched, long, and, 

 slender tubes which open into the intestinal canal of some of the higher 

 Crustaceans : these may be uriniferous tubes, like the corresponding 

 parts in certain insects and spiders. 



Tenacity of life in the Crustacea appears not to be enjoyed in any 

 unusual degree : but some species, as the Artemia salina, can exist 

 in hot and strong brine^ which would quickly destroy most other 

 animals. 



Like other Articulata subject to periodical ecdysis, the Crustacea 

 have the power of reproducing lost extremities. If a leg be fractured 

 or severed across one of its segments, it is cast oflf by a violent 

 muscular effort at the second articulation : if the Crustacean have 

 not the power of thus ridding itself of the wounded member, it 

 usually dies from the haemorrhage ; but this is immediately ar- 

 rested by the contraction of the lacerated part of the joint where 

 the limb is cast off" with least difliculty and pain. A small 

 cylindrical appendage first sprouts from the cicatrix, which soon after 

 presents distinct articulations, and resembles in miniature the limb 

 which it is destined to replace : its growth is slow until the period of 

 the moult, when, if the animal be in vigorous health, the new member 

 rapidly acquires its normal size. 



There is no propagation by spontaneous fission or gemmation in the 

 class Crustacea : every species is developed from ova formed by organs 

 peculiar to one series of individuals, and impregnated by the fertilising 

 product of organs as peculiar to another series. But although the 

 male and female organs are never naturally combined in the same in- 

 dividual, accidental or monstrous hermaphrodites occasionally occur, 



N 4 



