3o4 A HISTORY OF RECENT CRUSTACEA 
in swimming or climbing or clasping. The mouth-organs 
of one group are the grasping weapons of another. The 
walking legs of one set are elsewhere adapted for swim- 
ming. There are also other functions conjugal or maternal 
in which the swimming legs or the walking legs may take 
part, while the breathing apparatus, simple or complicated, 
may be connected with the mouth-organs or limbs of the 
trunk or both, or else with the swimming organs of the 
tail-part, commonly called the pleon. ‘These and other 
curious modifications are largely made use of in classifying 
the Crustacea, and to understand the unavoidable intrica- 
cies of any system of arrangement, the outgrowth of each 
segment should be studied, in some form the less abnormal 
the better. 
1. The first segment is known as the ocular or ophth- 
almic. This is clearly articulated in the Squillidee, but only 
occasionally in other groups, as in the macruran Plesi- 
onika uniproducta, Sp. Bate. Its imdividuality is m no 
way indicated in the sessile-eyed Crustacea, in some of 
which the eyes pretty well cover the whole dace surface 
of the head. In the Brachyura and Macrura its original 
independence can often be traced, but it is in these chiefly 
attested by the pair of ‘movable appendages which it 
almost invariably carries. Now, just as it is thought 
that all the segments represent a common original type 
variously modified, a similar view is applied to all the 
appendages that arise from the segments. 
The resemblance, in fact, is often very obvious between 
the antennz and the swimming feet ; between the laminar 
maxille near the mouth and the opercular and breathing 
plates in the tail; between the maxillipeds, which are in- 
struments of nutrition, and the ambulatory legs of the 
trunk; so that a connection, at first sight very improbable, 
1S satisfactorily established between them all, when the 
comparison has been sufficiently extended. Yet it de- 
mands some exercise of faith on the part of a novice to 
accept the declaration that this homology holds between 
the claw of a lobster and its eye. No two parts of an 
animal could well be more unlike in appearance and func- 
