. Elementary text-book of zoology. Zoology. 412 ABTHROPODA. The Crustacea, name is derived from the body 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 cephalothorax; which is followed b
. Elementary text-book of zoology. Zoology. 412 ABTHROPODA. The Crustacea, name is derived from the body 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 cephalothorax; 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 oft' 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 and 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 (Lemcece, Sacculina). On the head there are usually two pairs of antenna?, 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,
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
