ASTACUS. 125 



ASTACUS. 



Ix the genus Astacus the last tlioracic somite bears a gill (pleuro- 

 branchia) on each side, the full number of gills being thirty-six (eighteen 

 in each branchial chamber). There are besides two or three rudimentary 

 gills on each side of the body. The hindmost podobranchia is provided 

 with a plaited, bilobed lamina, like those in front. The arrangement of the 

 gills is expressed in the following formula : — 



Somite. I'oDoBr.ASCiii.E. AitrHiioBRAXCHi.E. I'LtuiioBRAXcm.E. 



AiUtii'ior. Posterior. 



YI. ... (ep.) .0...0...0 =0 (ep.) 



YII. ...1...1...0...0 =2 



VIII. ... 1 ... I ... 1 ... = .3 



IX. ... 1 ... 1 ... 1 ... =3 



X. ... 1 ... 1 ... 1 ... or r = 3 or 3 + r 



XL ... 1 ... 1 ... I ... »• =34-/- 



XII. .... 1 ... I ... 1 ... »• = S + r 



XIII, ...0...0...0...1 =1 



(5 _|_ ,.p. -(- G -|- .5 + 1 -|-2r or 3/' =: 18 + 2»-or3?- + ep. 



The orifice of the gi'een gland is situate on the posterior face of the tu- 

 bercle. The annulus ventralis is represented by a transverse ridge behind 

 the penultimate thoracic sternum. 



The Astaci occupy three widely separated geographical area^: 1. Western 

 North America from the Rocky Mountains to the Pacific Ocean;* 2. The 

 western portion of the Europteo-Asiatic continent, from the Ural Mountains 

 and the basin of the Sea of Aral to the Spanish peninsula and Ireland ; 

 3. Eastern Asia in the Amoor River system (Transbaikailia, Territory of 

 Amoor, and Manchooria), and in Japan. No Astaci are known from any 

 part of Siberia between Lake Baikal and the Ural Mountains, or from any 

 of the Siberian rivers that flow into the Arctic Ocean. t 



The North American and European Astaci form a natural group (Astacus 

 proper). In these the body is robust and ovate, the first pair of abdominal 



* One species, A. Gamhelii, has invaded the territory of the Canibari, following down the Yellowstone 

 River to its mouth. 



f That is, no species known to be indigenous. A. leptodactylits has been artificially introduced into the 

 Irtish River basin. 



