332 COMPARATIVE ANATOMY chap. 



We may assume, that in the homonomous segmented ancestors of 

 the Crustacea, whose conjectural organisation we have already in many 

 points diagrammatically sketched, 4 strongly -developed longitudinal 

 muscles ran through the body. Two of these muscles ran dorsally and 

 two ventrally on each side of the middle line. They were segmented 

 in correspondence with the segmentation of the body, and in such a 

 manner that the single muscle segments or myomeres lay interseg- 

 mentally, i.e. with one end attached to the integument of one segment, 

 and the other to the integument of the next following or next pre- 

 ceding segment. 



We have no difSeulty in assuming that the dorsal and ventral pair of longi- 

 tudinal muscle strands answer to the 4 similarly placed masses of longitudinal 

 muscles which occur in the Polyclueta. We can find nothing, certainly, con-espond- 

 ing with the circular musculature of the Annelida. Traces of the latter may 

 perhaps, however, be seen in the muscles which run at right angles to the longi- 

 tudinal muscles, and which are attached on the one hand to the integument of the 

 body, and on the other to the basal portion of the appendages which they move. 



Setting aside at first the muscles belonging to the inner organs, we can arrange 

 the whole musculature of the body in three principal groups, viz. (1) the muscu- 

 lature of the body, (2) the musculature of the limbs, and (3) the musculature common 

 to both. 



The formation and segmentation of the body and limbs in differ- 

 ent divisions varies so much in details that a comparative review of 

 the musculature cannot be attempted. We may, however, explain 

 the principle according to which the muscles are arranged, and per- 

 form their functions not only in all Crustaceans, but universally in all 

 Arthropoda. We have to bear in mind — (1) that the passive loco- 

 motory organ, the skeleton, is in the Arthropoda an exoskeleton, 

 which is in each segment of the body and in each joint of its extrem- 

 ities a ehitinous tube ; (2) that the muscles lie on the inner side of the 

 skeleton, and are attached to the same from within ; (3) that the 

 muscles are stretched intersegmentally, i.e. between the consecutive 

 segments ; (4) that the ehitinous integument between two segments is 

 thin and flexible, forming the interarticular membrane ; (5) that the 

 tubular exoskeleton of two adjacent segments or joints is hinged 

 together in each case at two transversely opposite points. 



The arrangement and working of the musculature is illustrated by 

 Fig. 230, which shows us 5 segments, one larger {d) and 4 smaller, in 

 vertical projection. The firm hard portions of the exoskeleton are 

 marked by strong outlines, the delicate and flexible interarticular 

 membranes {tg, sg) by dotted lines. The hinges between 2 consecutive 

 skeletal segments are marked a. Such a hinge is found on each side ; 

 in the projection the corresponding right and left hinges are seen as 

 one. A dorsal muscle {d) is attached to the larger segment {d), and 

 runs through the smaller segments, being inserted in the dorsal or 

 tergal skeleton (t) of each by means of a bundle of fibres. A ventral 

 muscle (v) does the same on the ventral or sternal side (s). 



