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, e.g., 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, a. 



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, v' vessel thence to the gill, a' from 

 the gill to the heart. — After Gegenbaur. 



better-developed system, and definite vessels, although these may 

 fail in many parts of the body, so that the blood flows in the 

 spaces between the organs ; the vascular system is consequently not 

 entirely closed. The circulation in gill-bearing Crustacea is as 

 follows (see Pig. 152) : The blood is driven from the heart, through 

 more or less perfect arteries into the different parts of the body ; after 

 it has received carbon-dioxide, and given up oxygen here, it collects 



