TYPE CRUSTACEA. 429 



showing but faint indication of segmentation, and bearing on 

 its terminal segment the anus, behind which is a long mova- 

 ble spine {sp), the post-abdomen, forming the third region 

 and to be regarded probably as a movable prolongation of 

 the dorsum of the last abdominal segment. 



The cephalothorax bears seven pairs of appendages. The 

 first pair, the cheUcerce (Fig. 195, ch), which lie in front of the 

 mouth, are small and, like the following four pairs, are chelate. 

 These together with the sixth are much longer and surround 

 the mouth, their basal joints being provided with strong bris- 

 tles and serving as jaws. The sixth pair of appendages differ 

 from their predecessors in not being chelate and in possessing 

 upon their basal joints a peculiar process which has been 

 termed the flabellum and by some is regarded as representing 

 an exopodite The seventh pair of appendages is very differ- 

 ent from the others, forming a broad flat plate, the two 

 appendages of the opposite sides meeting in the middle line. 

 This plate covers in the abdominal appendages to a certain 

 extent and hence is termed the opercidum (op). The abdom- 

 inal appendages, of which there are five pairs, resemble the 

 operculum in form, and like it allow an external larger exopo- 

 dite and an inner smaller endopodite to be distinguished. 

 They carry upon their posterior surfaces series of large leaf- 

 like, thin-walled folds which function as branchiae. 



The heart (Fig. 196, ht) is an elongated tubular organ lying 

 in the posterior part of the cephalothorax and the anterior 

 part of the abdomen, and possesses eight ostia. Arteries 

 arise from it which v carry the blood to various parts of the 

 body, eventually, however, opening into the general lacunar 

 system. The blood has a distinct bluish color M'hich deepens 

 on exposure to the air and is due to a copper-containing 

 respiratory pigment, hsemocyanin. 



The body is enclosed in a hard chitinous cuticle, and in 

 addition a peculiar fibro-cartilaginous plate, the endosternite, 

 is found in the cephalothorax between the intestine and the 

 nervous system. It is formed by the fusion of a number of 

 tendons and may be regarded as an endoskeleton. 



The mouth is an elongated opening lying between the 

 bases of the anterior cephalothoracic appendages and is 



