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CHAPTER XX 



CRUSTACEA 

 I 



PASSING to the division of Arthropods, in which the body is 

 furnished with distinctly articulated or jointed limbs, some of which 

 are always modified to serve as mouth-organs, we come first to the 

 class of Crustacea, which ordinarily includes (when used in its 

 most comprehensive sense) all those animals belonging to this group 

 which are fitted for aquatic respiration, though the king-crab 

 (Limulus) seems to have closer relations to the scorpions, and the 

 Pycnogonids to the spiders. It thus comprehends a very extensive 

 range of forms ; for although we are accustomed to think of the crab, 

 lobster, cray-fish, and other well-known species of the order Decapoda 

 (ten-footed) as its typical examples, yet all these belong to the highest 

 of its many orders ; and among the lower are many of a far simpler 

 structure, not a few which would not be recognised as belonging to 

 the class at all were it not for the information given by the 

 study of their development as to their real nature, which is far more 

 apparent in their early than it is in their adult condition. Many 

 of the inferior kinds of Crustacea are so minute and transparent 

 that their whole structure may be made out by the aid of the 

 microscope without any preparation ; this is the case, indeed, with 

 nearly the whole group of Entomostraca, and with the larval forms 

 even of the cra>, and its allies ; and we shall give our first atten- 

 tion to these, afterwards noticing such points in the structure of the 

 larger kinds as are likely to be of general interest. 



A curious example of the reduction of an elevated type to a 

 very simple form is presented by the group of Pycnogonida, or no- 

 body crabs, some of the members of which may be found by atten- 

 tive search in almost every locality where seaweeds abound, it 

 being their habit to crawl (or rather to sprawl) over the surfaces of 

 these, and probably to imbibe as food the gelatinous substance with 

 which they are invested. 1 The general form of their bodies (fig. 

 719) usually reminds us of that of some of the long-legged crabs, 

 the abdomen being almost or altogether deficient, whilst the head is 

 very small, and fused (as it were) into the thorax ; so that the last- 

 named region, with the members attached to it, constitutes nearly 

 the whole bulk of the animal. The head is extended in front into 



r T It is remarkable that very large forms, of this group, sometimes extending to 

 more than twelve inches across, have been brought up from great depths of the sea. 



