CHAPTER XVI. 



CRUSTACEA. 



PASSING from the lower division of the Articulated series to 

 the higher that in which the body is furnished with distinctly 

 articulated or jointed limbs, we come first to the class of Crus- 

 tacea, 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 struc- 

 ture 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 ( 366), and with the larval forms even of the 

 Crab and its allies ( 375) ; 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. 



365. One of the most curious examples of the reduction of an 

 elevated type to its very simplest form, which the Animal King- 

 dom affords, is presented by the group of Pycnogonidce ; 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 pro- 

 bably to imbibe as food the gelatinous substance with which they 

 are invested. The general form of their bodies (Fig. 275) usually 

 reminds us of that of some of the long-legged Crabs ; the abdo- 

 men 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 proboscis-like t projection, at the extremity of which is the 



