552 



CRUSTACEA. 



rable, in almost all essential particulars, to that of Cyclops (§ 367). 

 In the case of the Lobster, Prawn, and other " macrourous" 

 species, the metamorphosis chiefly consists in the separation of 

 the locomotive and respiratory functions, true legs being deve- 

 loped from the thoracic segments for the former, and true gills 

 (concealed within a special chamber formed by an extension of 

 the carapace beneath the body) for the latter; and the abdominal 

 segments increase in size, and become furnished with appen- 

 dages (false feet) of their own. In the Crabs, or " brachyour- 

 ous" species, on the other hand, the alteration is much greater; 

 for besides the change first noticed in the thoracic members and 

 respiratory organs, the thoracic region becomes much more 

 developed at the expense of the abdominal, the latter remaining 

 in an almost rudimentary condition, and being bent under the 

 body ; the thoracic limbs are m6re completely adapted for walk- 

 ing, save the first pair, which are developed into chelce or pincers ; 

 and the little creature entirely loses the active swimming habits 

 which it originally possessed, and takes on the mode of life 

 peculiar to the adult. We have, in this histoiy, a most charac- 

 teristic example of Von Bar's great law of " progress from the 

 general to the special" in organic development; for the Ento- 

 mostracous form is thus seen to be common to the highest and 

 the lowest Crustaceans in the earliest phase of their lives ; but 

 whilst the latter remain and go on to completion upon that type, 

 the former entirely diverge Irom it ; and whilst diverging from 

 it, they also become dififerentiated from each other, the distinc- 

 tive characters of their families, genera, and species, evolving 

 themselves, as the individuals advance towards their mature 

 forms. 



376. In collecting minute Crustacea, whether fresh-water or 

 marine, the use of the ring-net, as for minute Acalephse or 

 Echinoderm larvfe, will be found the most efiicient instrument; 

 and in favorable localities, the same "gathering" will often con- 

 tain multitudes of various species of Entomostraca, accompanied, 

 perhaps, by the larvae of higher Crustacea, by Echinoderm 

 larvae, by Annelid larvae, and by the smaller Medusae. The 

 water containing these should be put into a large glass jar freely 

 exposed to the light; and after a little practice, the eye will 

 become so far habituated to the general appearance and modes 

 of movement of these different forms of Animal life, as to be 

 able to distinguish them, one from the other. In selecting any 

 specimen for microscopic examination, the dipping tube (§ 71) 

 will be found invaluable. The study of the metamorphoses 

 of the larvae will be best prosecuted, by obtaining the fertilized 

 eggs which are carried about by the females, and watching the 

 history of their products. 



