XIV INTRODUCTION. 



cations of the thoracic appendages may be selected. The 

 typical structure of these may be considered as subserving 

 the purposes of locomotion. This is the office which they 

 fulfil, either wholly or in part, in all cases ; and in some 

 instances the whole of them are thus employed. In the 

 Isopoda^ for instance, the body consists very principally of 

 the seven thoracic segments, and their appendages consti- 

 tute seven pairs of true feet. In the Awiphipoda the first 

 or second pairs become modified in the male into strong 

 holders by the greater development of the hand, and the 

 movable character of the terminal articulation, and its ap- 

 plicability to a strong corresponding process from the 

 penultimate articulation. In several of the Lce.mocli'poda 

 five pairs only of the thoracic appendages are developed 

 into members, of which the first and second pairs consti- 

 tute true hands or graspers, and the third and fourth are 

 destined to a totally different office ; forming respiratory 

 sacs, to supply the place of the abdominal appendages in 

 the Isopoda, the abdomen in the present instance being 

 reduced to a mere rudiment. In the Decapoda there are 

 only five pairs of true thoracic members, and these answer 

 to the five posterior segments of the thorax ; but the ap- 

 pendages to the segments anterior to these are rendered 

 subservient to mastication, or to the preparation of the 

 food, in the form of footjaws or pedipalps. I have only 

 enumerated a few of the more conspicuous modifications 

 of these organs, for the purpose of conveying at a glance 

 some idea of the extraordinary aberrations from the typical 

 structure which will meet us at every step, in the inves- 

 tigation of these animals, whose habits and requirements 

 are so varied. 



The composition of the segments in the Crustacea, 

 although modified to a great extent in the different forms, 



