4 A HISTORY OF EECENT CRUSTACEA 



to one of these, tliey are ' AquaticArthropoda, which breathe 

 by means of gills. They have two pairs of antennge, 

 numerous paired legs on the thorax, and usually also on 

 the abdomen.' This is compendious and useful. The 

 statements clearly exclude all the other classes of the 

 Arthropoda. They are also widely applicable among crus- 

 taceans ; yet of these animals there are some which are 

 not aquatic, some which have no gills, some which have 

 not two pairs of antennge, and some in which the ' paired 

 legs on the thorax ' are not numerous. 



A different definition was given by Professor Alphonse 

 Milne-Edwards in 1860, according to which the class of 

 Crustacea comprises ' all the segmented animals with bran- 

 chial or cutaneous respiration, in which the body is pro- 

 vided with jointed limbs, whether permanent or transitory.' 

 The Insecta aud Myriapoda breathe by means of the air- 

 tubes called tracheae ; most of the Arachnida by means either 

 of tracheas or pulmonary sacs kuown as fan tracheae. From 

 all these, therefore, the definition separates the Crustacea in 

 a satisfactory manner, even though some terrestrial Crus- 

 tacea combine tracheate with branchial respiration. There 

 are, however, some subordinate members of the Arachnida, 

 and the whole class Pycnogonida, in which the respiration is 

 dependent on the surface of the body and not on any special 

 organs. As it is only in recent years that the Pycnogonida 

 have been constituted an independent class, it was no 

 fault of a definition framed in 1860, that it included them 

 among the Crustacea, to which they were then supposed to 

 belong. They are in fact separated by many characters, one 

 of which is the possession of a proboscis, which is supposed 

 to have originated in the coalescence of the upper lip and 

 the mandibles. So far as is known, they are all marine 

 animals. On the other hand, those Arachnida which have 

 surface-respiration are apparently all air-breathers. To 

 meet all existing requirements, then, the definition of the 

 Crustacea may be framed in the following manner : — 



They are Arthropoda without terminal proboscis, with 

 respiration branchial or cutaneous, the latter only aquatic. 



It is not to be expected that any legitimate definition 



