34 A HISTORY OF RECENT CRUSTACEA 



in swimming or climbing or clasping. The rnouth-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 Squillidas, but only 

 occasionally in other groups, as in the macruran Plesi- 

 oniJia uniproducta, Sp. Bate. Its individuality is in no 

 way indicated in the sessile-eyed Crustacea, in some of 

 which the eyes pretty well cover the whole dorsal 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 antennas and the swimming feet ; between the laminar 

 maxillaa 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, 

 is 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- 



