374 



THE CRAYFISH 



blood-sinus, and not as a true ccelome : it may therefore be 

 described as a hcemoccele. 



There are well-developed respiratory organs in the form 

 of gills (Figs. 91, g, 

 and 94, k\ contained 

 in a narrow branchial 

 chamber, bounded 

 internally by the ^ 

 proper wall of the 



t- 



cC~ 



OLDnC 



thorax, externally by 

 the gill-cover or 

 pleural region of the 

 carapace. Each gill 

 consists oT a stem 

 giving off numerous 

 branchial filaments, 

 so that the whole 

 organ is plume-like. 

 PThe filaments are 

 hollow and commu- 

 nicate with two 

 parallel canals in the 

 stem an external, 

 the afferent branchial 

 vein, and an internal, 

 the efferent branchial 

 veinj\g. 96). The 

 gill is to be considered 

 as an out-pushing of 

 the body- wall specially modified for respiration (compare p. 

 204), and it contains the same layers a thin layer of chitin 

 externally, then a single layer of epithelial cells, and beneath 

 this connective-tissue, hollowed out for the blood channels. 



FIG. 94. Transverse section of thorax of Crayfish ; 



diagrammatic. 



abut, ventral abdominal muscles ; bf. leg ; bm. 

 ventral nerve cord ; d. intestine ; dbm. dorsal 

 abdominal muscles; ep. wall of thorax ;/z. 

 heart ; k. gills ; kd. gill-cover ; /. digestive 

 gland ; ov. ovary ; pc. pericardial sinus ; sa, 

 sn. sternal artery ; i>s. ventral blood-sinus. 

 The arrows show the direction of the blood- 

 current. (From Lang's Comparative Ana- 

 tomy.) 



