412 



AET1IROPODA. 



The Crustacea, whose narae is derived from the Lody-covering 

 (which is often hardened), are principally aquatic animals. Some 

 forms, however, can live on land, and possess respiratory organs 

 adapted for breathing air. An important character of the group 

 is the great number of paired appendages. The appendages of all 

 the segments, even those of the head, may be used in locomotion 

 (fig. 330). As a rule, the head fuses with the thorax, or at any rate 

 with one or more of the thoracic segments, to form a cepludotliorax ; 

 which is followed by the remaining free thoracic segments. Some- 

 times, however, these two regions of the body remain distinct. The 

 head and thorax are seldom so sharply marked off from, one another 

 as, for example, in the Insecta : usually certain appendages, the 

 so-called maxillipeds, occupy an intermediate position between legs 

 and jaws, and being placed at the boundary between the two 



regions may be rec- 

 koned either as be- 

 longing to the head 

 or the thorax. The 

 fusion of the seg- 

 ments may be very 

 extensive ; not only 

 may the head arid 

 thorax be united, 

 but the boundary be- 

 tween thorax and 

 abdomen may vanish, 

 and the segmentation 

 may even disappear. As a general rule, the form of the body 

 presents extraordinary differences in the various groups. A redupli- 

 cature of the skin arching over the thorax and covering the body as 

 a shell is frequently present. This fold of the integument constitutes, 

 in extreme cases, a mantle-like investment, which may develop 

 calcareous plates and occasion a certain resemblance to Lamelli- 

 branchs (Cirripedia). In other cases the body has quite lost its 

 segmentation, and the animal resembles a worm (Lernccce, Sacculina). 

 On the head there are usually two pairs of antennae, which 

 function as sense organs and sometimes also as organs of locomotion 

 or of prehension. There is a pair of large jaws (the mandibles'), one 

 on each side of the mouth, over which a small plate, known as the 

 upper lip, often projects. The mandibles are simple but very rigid 

 and hard masticating plates, which are usually toothed and correspond 



Fl3. 330. Gammarus nerjhctus (after G. O. Sars). A', A", 

 The two antennae ; Ef, maxilliped ; F F\ first to seventh 

 thoracic feet ; Sf, anterior swimming feet. 



