CIRCULATION AND RESPIRATION OF CRUSTACEA. 



239 



multitude of small tubes terminating in follicles, and grouped 

 round a ramified excretory canal, whose extremity empties 

 itself on each side into the intestine near its commencement 

 (fo. Fig. 572). 



852. We know as yet nothing of the manner in which the 

 chyle passes from the intestine into the circulating apparatus. 

 The blood is colourless, or slightly tinged with blue or lilac ; and 

 coagulates easily. This liquid is put in motion by a heart, 

 situated on the median line of the back, and composed of but 



FIG. 573 CIRCULATING APPARATUS OF LOBSTER ; a, heart ; 6 and c, arteries to the 

 eyes and antennae ; d, hepatic artery ; e and /, arteries to thorax and abdomen ; gg, 

 venous sinus ; A, gills ; i, branchial veins. 



a single cavity of variable form (Fig. 573, a). Its contraction 

 propels the blood into the arteries, which distribute it to all 

 parts of the body. The veins are very incomplete, and are 

 formed chiefly by passages left between the different organs, and 

 lined by a thin layer of areolar tissue ; they end in large cavities 

 near the base of the legs (Fig. 574, *), from which the blood is 

 conducted to the respiratory organs, whence it returns to the 

 heart by very distinct canals termed branchio-cardiac vessels. 



853. Crustacea are almost all essentially aquatic; their 

 respiration is nearly always effected by gills; and when these 

 organs are absent, their place is supplied by the skin of certain 

 parts of the body, generally of the legs. In other respects, 

 the arrangement of the respiratory apparatus varies consider- 

 ably. Thus in the Crabs, Cray-fish, and all other Crustacea 

 of analogous organisation, the gills consist of a considerable 



