ASTACUS FLUVIATILIS. 281 



This anterior portion of the intestine is, however, very 

 short, and almost immediately becomes dilated into the wider 

 posterior division, which extends to the anus. The inner 

 surface of the dilatation is produced into six ridges, which are 

 continued into a corresponding number of series of papillae 

 along the rest of the intestine. 



The only glandular apparatus of any kind which opens 

 into the alimentary canal is the liver, and the apertures of 

 the wide hepatic ducts are seen on each side of the pylorus. 

 Each duct conveys the secretion from the multitudinous 

 ciecal tubes, which constitute the principal mass of the cor- 

 responding bilobed half of the liver. The two halves lie on 

 each side of the stomach, and, though they remain perfectly 

 distinct from one another, come into close contact below. 



AstacKS possesses neither salivary glands nor any csecal 

 appendages to the intestine, such as exist in the Brachyura 

 and some Macrura^ unless the short caecum just now de- 

 scribed is the homologue of the longer casca of Maia and 

 Ilomarus. 



In the spring and summer two very curious discoidal cal- 

 careous plates, the so-called ''eyes" of the Crayfish, are 

 found imbedded in the walls of the dilated anterior portion 

 of the cardiac division of the stomach, the middle of the 

 lateral surface of which they occupy. These bodies com- 

 mence as calcareous deposits underneath the chitinous gas- 

 tric lining, and increase in size until the period arrives at 

 which the Crayfish casts its skin. They are then cast, to- 

 gether with this lining membrane and the gastric armature ; 

 and it would appear that, like the latter, they become broken 

 up and destroyed within the new stomach. The purpose of 

 these concretions is not understood ; the ordinary theory, 

 that they are stores of calcareous matter, ready to be dis- 

 tributed through the young integument after ecdysis, appear- 

 ing to be negatived by their small size. Oesterlen slates 

 that they rarely weigh more than two grains, and judiciouslj^ 

 suggests that if it be admitted that the Crayfish can derive 

 all the calcareous matter it requires, except two grains, fiom 

 other sources, it is hardly necessary to look on those two 

 grains as a special supply. 



The circulatory apparatus of AsUtcus is well developed. 

 The heart (Fig. 74, C) has the shape of an irregular polygon, 

 and lies immediately behind the stomach and beneath the 

 cardiac region of the carapace, in a chamber which is com- 

 monly termed the "pericardium," to the walls of which it is 



