. Text book of zoology. Zoology. Class 1. Crustacea. 191 this function. In others, again, certain parts of the body are specially- developed as gills. Sometimes the thin-skinned inner side of the carapace serves as a breathing apparatus; in other cases, the flattened epipod, or some other part of the limb, acts as a gill; in yet others, the gills are special, usually branched, appendages, springing from the limbs or from the side of the body. These true gills, which are usually characterised by the possession of a close and delicate vascular network, may be supplemented by either the whole, or
. Text book of zoology. Zoology. Class 1. Crustacea. 191 this function. In others, again, certain parts of the body are specially- developed as gills. Sometimes the thin-skinned inner side of the carapace serves as a breathing apparatus; in other cases, the flattened epipod, or some other part of the limb, acts as a gill; in yet others, the gills are special, usually branched, appendages, springing from the limbs or from the side of the body. These true gills, which are usually characterised by the possession of a close and delicate vascular network, may be supplemented by either the whole, or a part, of the surface of the body. Some of tte teiTestrial Crustacea breathe atmospheric air, and here, peoiliar respiratory organs are sometimes developed. This is, , the case in Birgus latro, an East Indian form related to the Hermit-crab; the giUs are veiy small,, but the branchial-cavity, enclosed by the sides of the carapace, serves as an air-breathing oi-gan, and is, therefore, provided with large vascular excre- scences of the sm-face, arising from the inner side of the carapace. In a true hennit-crab, Ccenobiia, the soft skin of the abdomen sei-ves as a respiratory organ, and is furnished with a vascular network for this pm-pose. In some teiTestrial Isopods the abdominal limbs have branched invaginations of the skin which take in au- and serve as breathing apparatus. The vascular system exhibits a variety of modifications. In some forms, it is represented only by the heart, which drives the blood into the spaces between the organs; in a few cases, even this is wanting. A poorly-developed vascular system is usually correlated with a small body, and with the absence of special respiratory organs. When these are present, there is, as a rule, Fig. 152. Vascular system of the Lobster, diagrammatic; the Teasels which carry arterial blood, light, the others, dark ; the arrows indicate the direction of the blood stream, g gills, h heart, p pericardium, v venous sinus,
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