43 REPOKT — ]875. 



out and a nomouclaturc suggested which, it was hoped, would to a largo extent 

 overcome the great difficulty in the study of this branch of natural history. 



But although many of the terms there given have become very general in 

 use, yet the custom of some writers of applying different ones at separate 

 times for the same parts is significant of a confusion of ideas that precludes 

 the student from a just appreciation of the labours of others. 



I do not think that this difficulty will be overcome for some long period 

 unless a committee is appointed by this Association, consisting of all the best 

 known authors of carcinological works, who shall determine upon a syste- 

 matic nomenclature for the structure and classification of the Crustacea to 

 which all future writers shall conform. 



In this Eeport I purpose provisionally, except when quoting from others, to 

 make use of the same terminology as that adopted in the previous Ileport, and 

 confine each term to that which has homologically the same signification. 



In the classification of Crustacea iu his great work*, Dana states 

 that " in the crustacean type there are normally twenty-one segments, 

 and correspondingly twenty-one pairs of members, as laid down by Slilne- 

 Edwards, the last seven of which pertain to the abdomen (ijleon) and the first 

 fourteen to the cephalothoras (cephalon and pereion). Now we may gather 

 from an examination of the crab, or macrural decapod, acknowledged to bo 

 first in rank, what condition of the system is connected with the highest 

 centralization in Crustacea. 



" In these highest species, nine segments and nine pairs of appendages out 

 of the fourteen ce2}JinIothor cede helong to the senses and mouth, and five pairs 

 are for locomotion. Of these nine, three are organs of senses, six are mandibles 

 and maxillaj." 



M. Milne-Edwards, in his standard work ' Histoire Naturelle des Crustaccs,' 

 says, " We can generallj^ distinguish among these animals a head, a thorax, 

 and an abdomen ; but the limit of these regions is not always naturally well 

 defined ; and it is not well to attach too much importance to these distinctions, 

 for they do not correspond vrith the same parts among mammals, birds, &c. 

 . . . ." And in a note to the above he says, " Guided by the principal viscera 

 some authors have given the name of abdomen to the thorax, and that of 

 postabdomen to that which we call abdomen ; but after this principle we 

 must consider the head to be a preabdomen, because it contains the same 

 viscera as the thorax and abdomen." 



The twenty-one somites of the t3rpical Crustacea M. Milne-Edwards has 

 thvis divided — the anterior seven to the head, the next seven to the thorax, 

 and the posterior seven to the abdomen. Eut in his nomenclature of the 

 appendages the terms used are suggestive of the anterior two pairs of the 

 thorax being attached to the head. In his " Observations sur le Squelette 

 tegumentaire des Crustaces decapodes," Ann. des Sciences, 1854, the same 

 author statesthat "he has often been convinced thatin many branches of zoology 

 the difficulties of the study are considerably augmented by the imj^erfeetion 

 of the language by which we attempt to formulate the results of our observa- 

 tions. The employment of expressions that are vague in the determination of 

 zoological characters and the description of the parts that constitute an 

 organism convey naturally the superficial observation with which the observer 

 was content, leaving in the mind of the reader an amount of doubt which 

 retards his desire for distinct information The terms," he con- 

 tinues, " of zoology are far, at present, from that degree of precision 



These considerations have determined me to make a general revision of the 

 * United States' Exploring Expedition, p. 1397. 



