334 



COMPA HA TI I 'E A NA TOM 1 ' 



CHAP. 



body, as we see from the movements of the abdomen of the Cray-fish (Figs. 231 and 

 232). In its normal position the abdomen is somewhat stretched, and lies more or 

 less as a straight posterior prolongation of the cephalo-thorax. Each dorsal (tergal) 

 skeletal segment (Fig. 231, A, t, ti, t. 2 ) has its most anterior somewhat thinner portion 



(b-c, 



pushed some distance under the posterior edge of the preceding segment. 



The iuterarticnlar membrane (c-a v Ci-a. 2 ) being 

 bent backwards and outwards. The ventral 

 hard skeleton consists of relatively narrow 

 transverse segmental stripes (d-c, d : -c v d-2-c. 2 ), 

 connected by large intersegmental membranes 

 (e-d l} e 1 -d- 2 ), which in a state of rest are 

 somewhat stretched. The pair of dorsal 

 muscles (tni) are attached on the one hand 

 anteriorly to the lateral walls of the cephalo- 

 thorax (corresponding with the large segment 

 of our diagram, Fig. 230), and inserted, on 

 the other hand, by just as many pairs of 

 bundles as there are abdominal segments, to 

 the inner surfaces of the terga, a pair of 

 muscles being inserted in each. 



The pair of ventral or sternal muscles (sm) 

 are attached anteriorly to the ventral side of 

 the cephalo-thorax to a row of processes of 

 the exoskeleton directed inwards, and partly 

 bound together by transverse ridges, which 

 roof over the thoracic portion of the ventral 

 chord and the sub-neural vessel. Posteriorly 

 the sternal longitudinal muscles are succes- 

 sively inserted on the inner sides of the sternal 

 skeleton of the abdominal segments. The 

 fibres of this muscle are twisted like the 

 strands of a rope. 



If the pair of sternal or flexor muscles con- 

 tracts, the abdominal row of segments cor- 

 responding with our diagram bends ventrally 

 (Fig. 231, ) till, as is the case in swimming, 

 the telson touches the ventral side of the 

 cephalo-thorax. While in this position the 

 intersternal membranes of the abdomen are 

 folded, the whole tergal integument is 

 stretched, and the tergal articular facets and 

 interarticular membranes are drawn out from 

 under the terga which cover them, and 



FIG. 232. Astacus fluviatilis. Longi- 

 tudinal section to represent the most 

 important muscles and their relation to 

 the exoskeleton (after Huxley), em, Ex- 

 tensor ; fm, flexor of the abdomen ; u<lm, 

 adductor of the mandibles 

 thorax; ab]-abg, abdominal 

 telson ; 1-19, limbs, 1-13, of the cephalo- 

 thorax, 14-19, of the abdomen. 



ctJi, cephalo- come freely to the surface. If now the sternal 

 segments ; t, musc le slackens, and the pair of tergal or 

 extensor muscles (Fig. 231, A ; Fig. 232) con- 

 tracts, the abdomen is straightened, the terga 

 pass under one another like the tiles of a roof, and the intersternal membranes are 

 stretched. A dorsal bending of the body is impossible, first because the intersternal 

 membranes do not allow of further stretching, and secondly, because the terga can 

 only be pushed one beneath the other as far as the posterior limit of their articular 

 facets. This is best exemplified by the illustration (Fig. 231, A}. 



