63G 



CHAPTER XVI. 



CRUSTACEA. 



Passing from the lower division of the Articulated series to the 

 higher — that in which the body is furnished with distinctly-articu- 

 lated or jointed limbs, — we come first to the Class of Crustacea, 

 which includes (when used in its most comprehensive sense) all 

 those animals belonging to this group, which are fitted for Aquatic 

 Respiration. 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, and not a few which would not be recognized 

 as belonging to the Class at all, were it not for the information 

 derived from 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 (§ 497), and 

 with the Larval forms even of the Crab and its allies (§ 508); and 

 we shall give our first attention to these, afterwards noticing such 

 points in the structure of the larger kinds as are likely to be of 

 general interest. 



496. A curious example of the reduction of an elevated type to 

 a very simple form is presented by the group of Pycnogonidm; some 

 members of which may be found by attentive search in almost 

 every locality where Sea-weeds 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. The general form of their bodies (Fig. 330) 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 a 



