CRUSTACEA MUSCULATURE 



333 



The skeletal segments may be compared to a double-armed lever 

 whose fulcrum lies in the hinges. If the dorsal muscle contracts, it 

 draws the dorsal arm of the lever (the tergal portion of the skeleton) 

 in the direction of the pull towards the larger segment ; the tergal 

 interarticular membranes become folded, the ventral stretched, and 

 the 4 segments bend upwards (Fig. 230, A). If the ventral muscle 

 contracts, while at the same time the dorsal slackens, the row of 

 segments will be bent downwards (Fig. 230, C). 



It is evident that the same effect would be attained if the dorsal 



FIG. 231. Two abdominal segments of the Cray-fish, diagrammatic, t, ti, t%, Tergal ; s, s^ s%, 

 sternal exoskeleton ; x, hinges ; b-c, t>i-ci, articular facets, which when the row of segments is 

 straightened take the position shown in B; ai-bi, a 2 -&2, c-aj, Ci-a 2 , e-di, ej-d 2 , interarticular 

 membranes ; tm, tergal ; sm, sternal longitudinal muscle. A, Row of segments nearly straightened ; 

 B, Row of segments bent ventrally by the contraction of the sternal longitudinal muscle, 

 stretching the intertergal membranes. 



and ventral muscles, instead of collectively running from the smaller 

 segments to the larger segment, only ran from one segment to the next 

 following, as we assumed to be the case in the racial form, and as is 

 really the case in the thorax and abdomen of many Crustaceans (Branchi- 

 pus, many Isopoda, Amphipoda, etc.) The row of segments would then 

 likewise bend dorsally on the contraction of the dorsal myomeres, and 

 ventrally on the contraction of the ventral. 



In reality only the ventral bending takes place in the Crustacea ; the compara- 

 tively weakly developed dorsal longitudinal muscles serve only for straightening the 



