CRUSTACEA. 205- 



CHAPTER XVIII. 

 CRUSTACEA. 



601. PASSING from the lower division of the Articulated series to 

 that of Arthropods, in which the body is furnished with distinctly articu- 

 lated or jointed limbs, we come first to the Class of Crustacea, which in- 

 cludes (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 accus- 

 tomed 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 ( 603), and with the larval forms even of the- 

 Crab and its allies ( 614); 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. 



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

 very simple form is presented by the group of Pycnogonida, some of the 

 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. 1 The general form 

 of their bodies (Fig. 410) 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 

 probosis-like projection, at the extremity of which is the narrow orifice 

 of the mouth; which seems to be furnished with vibratile cilia, that serve 

 to draw into it the semi-fluid aliment. Instead of being furnished (as in 

 the higher Crustaceans) with two pairs of antennas and numerous pairs of 

 'feet-jaws/ it has but a single pair of either; it also bears four minute 

 ocelli, or rudimentary eyes, set at a little distance from each other on a 



1 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. 



