20 



CRUSTACEA. 



changes of form in the existing parts as may accompany either of 

 these conditions. 



The normal parts of the separate rings or segments in the Articu- 

 lata should be in mind in tracing out the homological relations of 

 species, that is, that each of these rings consists normally of eight parts 

 or segments, two below, called sternal, two above, called dorsal, one 

 either side of the sternal, called the episternal, and one either side of 

 the dorsal, called epimeral. 



A different typical structure has been recently suggested, according 

 to which the parts are multiples of the number six, instead of serai. 

 It is based on the supposition that the organ called* the posterior or 

 lower lip by Edwards and others, is a true pair of maxillae, to be 

 counted with the following organs. Admitting this as correct, the 

 cephalothorax consists normally of fifteen segments ; the first three, 

 organs of senses, the next twelve pertaining to the mouth and thorax, 

 the last six of these twelve (the outer maxillipeds thus included) being 

 properly thoracic, according to the hypothesis. The abdomen, accord- 

 ing to this hypothesis, consists of six segments, bearing appendages, 

 and a seventh, which is normally composed of three segments. This 

 gives for the normal number of segments twenty-four, a multiple of 

 three or six. 



As the truth of this hypothesis is to be ascertained mainly by in- 

 quiring whether the so-called lower lip corresponds to a pair of maxillas 

 or not, we offer a few considerations on this point. The organ consists 

 of two oblong flat lobes, in some Decapods, somewhat maxilla-like in 

 form. We observe, in the first place, that it is peculiar in being with- 

 out articulations, and in no species throughout the range of Crustacea 

 does it bear a palpus, or any corresponding appendage. In these par- 

 ticulars it is unlike true maxillae. In some Schizopoda, as the Euphau- 

 sia, it is a small, quadrate plate, consisting of two naked lobes in con- 

 tact on the medial line ; and descending lower among Crustacea, the 

 organ is a simple plate, with the lobes quite short and small. In the 

 Caligi, the part corresponding to the lower lip forms the lower or pos- 

 terior part of the buccal trunk. Such are in general the variations it 

 undergoes. 



Very different are the variations among the other mouth-organs. 

 While in the Decapoda the lower lip is comparatively larger than in 

 the Entomostraca, the maxillaj of the former are comparatively smaller 

 than in many of the latter. Among the Entomostraca, these organs 



