332 THE CRAYFISH less, xxvii 



It will be seen that the body-muscles of Astacus cannot 

 be said to form a layer of the body-wall, as in Polygordius, 

 but constitute an immense fleshy mass, filling up the greater 

 part of the body-cavity, and leaving a very small space 

 around the enteric canal. 



In the limbs (Fig. 85) each podomere is acted upon by 

 two muscles situated in the next proximal podomere. These 

 muscles are inserted, by chitinous and often calcified 

 tendons, into the proximal edge of the segment to be 

 moved, the smaller {exi) on the extensor, the larger {fl) on 

 the flexor side, in each case half-way between the two 

 hinges, so that a line joining the two muscular insertions is 

 at right angles to the axis of articulation. 



The digestive organs are constructed on the same general 

 plan as those of Polygordius, but present many striking 

 differences. The mouth (Fig. 83, a, Mf/i) lies in the middle 

 ventral line of the head, and is bounded in front by a shield" 

 shaped process, the labrum, at the sides by the mandibles, 

 and behind by a pair of delicate lobes, i\\Q paragnatha. It 

 leads by a short wide gn/let (Fig. 83, Gui ; Fig. 86, cb) into 

 a capacious *' stoviach" which occupies a great part of the 

 interior of the head, and is divided into a large anterior or 

 cardiac division (Fig. 83, St ; Fig. 86, cs), and a small pos- 

 terior or pyloric division (ps) : the latter passes into a narrow 

 and very short small intestine (Fig. 83, S. hit ; Fig. 86, nid), 

 from which a somewhat wider large intestine (Fig. 83, Z. 

 Int ; Fig. 86, hd) extends to the anus {an), situated on the 

 ventral surface of the telson. 



The outer layer of the enteric canal consists of connective 

 tissue containing striped muscular fibres : within this is a 

 single layer of columnar epithelial cells, none of them 

 glandular. In the gullet and stomach, and in the large 

 intestine, the epithelium secretes a layer of chitin, which 



