ON OUR PRESKNT KNOWLEDGE OF THE CRUSTACEA. 43 



' carcinological terminology ' before presenting to zoologists the work that has 

 engaged me for some time on the natural distribution of Crustacea from the 

 collection in the natural-history museum." 



Even after this M. Milne-Edwards uses the terms head, thorax, and 

 abdomen, which he had previously stated to be "regions not naturally defined," 

 and gives the api)ellation of pemptorinatlie and hectO(/nathe to the first and 

 second pair of appendages attached to the thorax (or pereion). Dana made 

 his researches on the highest form in crustacean life ; so also has M. Milne- 

 Edwards in his later observations. Buttho two appendages which this latter 

 author determines as the seventh and eighth pairs of gnathes are invariably, 

 according to his own showing, the anterior two pairs of the thorax. It is 

 only in the highest and most consolidated form of crustacean life that we 

 find them variated from their typical character so as to make them appear 

 organs attached to the mouth ; whereas in a very considerable proportion of 

 the various forms of Crustacea they never act as attendants on the mouth, 

 but are simply prehensile in their character or locomotive in their power : 

 but almost universally throughout Crustacea they are connected with a pair 

 of branchial appendages ; and in this they fulfil most efficient work, so that in 

 the highest types their connexion with the mouth is one of secondary impor- 

 tance only. 



The first two pairs of appendages belonging to the pereion (or thorax), 

 through nearly all the orders, of the typical crustacean exhibit a variation 

 that distinguishes them from those posterior to them ; and it may be convenient 

 to define them, but certainly not by a term that confuses them with appen- 

 dages that are only connected with secondary duties. 



Taking into consideration the many and various forms of Crustacea, the 

 great and numerous changes they undergo, it is desirable not only to be sure 

 that the nomenclature shall be scientifically correct in its determination and 

 homological signification, but that it is convenient and applicable to a very 

 considerable proportion of the animals it has to define. 



A typical crustacean in any of the well-defined orders can readily be 

 divided into three parts, each part to consist of seven somites. 



Thejirst division -we call the cephalon*. It consists of the anterior seven 

 somites, and supports the organs of sense and appendages adapted to bo 

 attendants on the mouth. 



The second division we call the pereiok. The seven somites that form 

 this division support appendages that are more or less adapted for walking in 

 their most normal condition. 



The third division we name the pleon. This consists of the posterior 

 seven somites ; these support the appendages which, when developed, are always 

 more or less perfectly adapted for swimming. 



The last somite of the pleon is almost universally variated from the others, 

 and is developed much to resemble an appendage itself. It is, however, the 

 posterior somite, and as such we designate it by the name of the telson. 



The appendages that are attached to these several divisions are known by 

 their relation to them. Those that are connected with the senses are deter- 

 mined by their character — such as the eyes, antennae, and oral appendages. 



The antennae may be distinguished as the anterior and posterior pair, or as 

 the auditory or olfactory respectively, in preference to that of the inner and 

 outer or upper and lower, which is liable to vary. So the fourth pair of 

 appendages, or the first belonging to the oral group, may be known (from their 

 mandibular power) as the mandibles, while the three following may be detcr- 

 * For the derivation of these terms see Beport of the British Ednophthahnia, 1855. 



