36o 



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 gills (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-cover or 

 pleural region of the 

 carapace. Each gill 



^"J^hkI,^^^^^ \^~^/J/^/i consists of a stem 



^BKMS^^^^t \ '^7^ iJ 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 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. 87. — Transverse section of thorax of Crayfi..,h, 

 dtagrammatic. 

 abni. ventral abdominal muscles; I'J. leg; bm. 

 ventral nerve cord ; d. intestine ; dbm. dorsal 

 abdominal muscles; ep. wall of thorax;/?, 

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

 glands ; im. ovarj' ; pc. pericardial sinus ; sa, 

 sn. sternal artery; vs. ventral blood-sinus. 

 The arrow shows the direction of the blood- 

 current. (From Lang's Comparative Ana' 

 toniy.) 



