200 ARTHKOPODA [CH. 



human liinb where the skeleton is internal. In each segment of each 

 limb (except the most distal) there are four muscles, two flexors and 

 two extensors connecting it with its predecessor and successor, but 

 the plane in which any one segment can move on its successor is 

 different from that in which it moves on its predecessor ; in fact, 

 there is a different plane of movement between every pair of segments, 

 so that by the combined movement of every joint the limb can be 

 bent into any position. [The construction of the crayfish's limb 

 has been imitated by a firm of American instrument makers who 

 have produced a lens-holder on this plan which can be bent into any 

 shape.] The muscles which move the mandibles are very powerful, 

 the adductor which brings them together forming a great fleshy 

 mass at the side of the stomach. Each abdominal segment is 

 connected with the next by a pair of flexors and extensors, and 

 since the downstroke is, as we have seen, the motion by which the 

 crayfish swims backwards the flexors form a very much thicker bed 

 of flesh than do the extensors. The intestine lies above the flexors 

 and below the extensors. From the first segment of the abdomen 

 extensors and flexors pass into the thorax, in each case breaking up 

 into several slips of muscle which are inserted in different segments 

 of the thorax, indicating that the segments of the thorax, though 

 now immovably fused with one another, were once movable. The 

 cells of which the muscles are composed are very different from 

 the muscle-cells described in the Annelida. Muscle-cells of that 

 description are, it is true, found in the muscular walls of the heart 

 and arteries and intestine of the crayfish, but the muscles by which 

 the quick movements of the body-segments and appendages are 

 carried out are composed of muscle-cells of a more complex kind. 

 These are much longer than the spindle-shaped muscle-cells of 

 Lumbricus, and are clothed with a delicate cuticle called the sarco- 

 lemma; they each have numerous nuclei which are embedded in 

 unmodified protoplasm, and the remainder of the protoplasm is 

 converted into contractile fibrils. These fibrils are of a composite 

 nature, being made up of alternate discs of a lighter, apparently 

 semi-fluid, material and a darker solid doubly -refracting material. 

 Each fibril is divided into a series of segments or sarco styles, each 

 sarcostyle consists of a disc of doubly refracting or anisotropic 

 substance with a disc of singly refracting isotropic substance on 

 each side. Each sarcostyle is separated from the next by a thin 

 membrane, " Krause's membrane." When contraction occurs the 

 isotropic substance is absorbed by the anisotropic which swells and 



