HOMOLOGIES OF CRUSTACEA. 21 



and the mandibles as well as the maxillipeds are often enlarged into 

 feet, the palpus being much lengthened into a pediform or natatory ap- 

 pendage. But this lower lip retains its fold-like character and par- 

 takes of none of these modifications, being the same essentially in the 

 highest and lowest species, excepting a diminution in size in the 

 latter. The mandibles and maxillae it will be remembered form a con- 

 tinuous series, alike in their relations and similar in their modifica- 

 tions : while the lower lip, although following next after the mandibles, 

 undergoes no corresponding variations. 



It is obvious, therefore, that this organ, which is never jointed, 

 never developes a palpus, and never takes a pediform character, is not 

 a member of the same series with the mandibles, maxillae, and feet, 

 and that, in fact, it is only a fold of the skin, as generally understood. 



Excluding this organ as only a lower lip, as done by Edwards and 

 others, we have the number of segments for the cephalothorax, just 

 fourteen, and those of the abdomen, seven. This number may be 

 actually counted in some species. The idea that the last abdominal 

 segment consists normally of three segments, cannot be inferred from 

 observation. The teeth of the margin are no necessary indication of 

 such sutures, no more than are those of the carapax. 



In the legs of the higher Crustacea, the number of joints is six. 

 But this number becomes seven if we count the episternal plate which 

 belongs to each, and which sometimes admits of some motion. 



2. Subdivision of the Body into Cepluilothorax and Abdomen. Before 

 proceeding further, it is necessary to consider the actual natural group- 

 ing of the parts in Crustacea. May we distinguish three separate 

 sections to the body, as in Insects, a head, thorax and abdomen or 

 only two, the head and thorax being united in one, and the abdomen 

 the other ? The latter is the accepted and true view. Crustacea have 

 a cephalothorax, but not a head ; and even in the very few species 

 which have a separate antennary segment in front, it is rather an un- 

 usual development of the anterior portion of the body, than any new 

 fundamental subdivision, for this anterior part does not include any 

 of the mouth-organs, some of which are cephalic organs in all animal 

 species that are said to have a head. The principal arguments ap- 

 pealed to, as proof of the oneness of the cephalothorax, are as fol- 

 lows : The continuous succession of parts in the cephalothorax, and 

 the absence of any constriction dividing a thoracic portion from a 



6 



