374 



THE CRAYFISH 



CHAP. 



upon as a large blood-sinus, and not as a true coelome : 

 it may therefore be described as a hcemoccde. 



There are well-developed respiratory organs in the 

 form of gills (Figs. 

 91, g, and 94, ^con- 

 tained in a narrow 

 branchial chamber, 

 bounded internally 

 by the proper wall 

 of the thorax, ex- 

 ternally by the gill- 

 cover or pleural 

 region of the cara- 

 pace. Each gill 

 consists of a stem 

 giving off numerous 

 branchial filaments, 

 so that the whole 

 organ is plume-like. 

 The 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 

 vein (Fig. 96). The 

 gill is to be con- 

 sidered as an out- 

 pushing of the body- wall specially modified for respiration, 

 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, (x about 4.) 

 abm. ventral abdominal muscles ; bf. leg ; bm. 

 ventral nerve-cord ; d. intestine ; dbm. dorsal 

 abdominal muscles ; cp. wall of thorax ; h. 

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

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

 sn. sternal artery ; vs. ventral blood sinus, 

 The arrows show the direction of the blood- 

 current. (From Lang's Comparative Ana- 

 tomy.) 



