j6o 



THE CRAYFISH 



lined by epithelium, and is to be looked upon as a large 

 blood-sinus, and not as a true ccelome. 



There are well-developed respiratory organs in the form 



of gi//s (Figs. 84, g, 

 and 87, k), contained 

 in a narrow branchial 

 chamber, bounded 

 internally by the 

 proper wall of the 

 thorax, externally by 

 the gill-c over or 

 pleural region of the 

 carapace. 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. 89). 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 la}ers — a thin layer of chitin 

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

 this connecti\e tissue, hollowed out for the blood channels. 



Fig. 87, 



Transverse section of thorax of Craylisli, 

 diagrammatic. 

 but. ventral abdominal muscles ; I^/. leg ; brn. 

 ventral nerve cord ; d. intestine; dbiu. dorsal 

 abdominal muscles ; e/>. wall of thorax ; //. 

 heart; tc, gills; kd. gill-cover; I. digestive 

 glands ; oz'. ovary ; pc. pericardial sinus ; sa, 

 sn. sternal artery ; 7's. ventral blood-sinus. 

 The arrow shows the direction of the blood- 

 current. (From Lang's CoJiiparativc Ana- 

 tomy.') 



