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PJiOCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 39, 



They are followed by other similar muscles (n), approximately par- 

 allel to the median line and close to it on either side. These are 

 arranged in series, three pairs extending from each segment of the 

 thorax forward into the preceding segment, from whose dorsal sur- 

 face they originate. They produce flexion of one segment upon 

 another. 



There is a single set of similar muscles (s) in the abdomen, extend- 

 ing from the anus forward into the genital segment, where they are 

 attached to the dorsal surface of the latter. 



This indicates that while the abdomen may be divided into seg- 

 ments by grooves, it nevertheless operates as a single joint, and there 

 is no flexion between its parts. 



For moving the swimming legs there is a set of two muscles, or 

 rather two bundles of muscles, (m) extending out from the median 

 line to the basal joint of each leg. The posterior bundle extends 

 outward from the median line itself and nearly at right angles to it. 

 The anterior one is more inclined to the body axis and seldom reaches 

 the center of the segment. 



In the case of the first pair of legs both muscles are diagonal (r), 



and extend in but a very little ways toward the center of the segment. 



There are finally in the genital segment the muscles which control 



the genital openings (fig. 3). These openings are irregular in shape 



and are situated on the dorsal or partly on the 

 lateral surface of the segment, near its posterior 

 end. Each is surrounded by a thickened and 

 somewhat chitinous border, which has the shape 

 of a much twisted letter V. The point of the V 

 is close to the posterior corner of the segment, 

 while the arms extend forward. Each arm is 

 bent inward near its center toward the median 

 line and the inner ends are enlarged. The pos- 

 terior arm extends farther forward before bending 

 and is bent almost at a right angle, whose terminal 

 branch is considerably shorter than the basal. 

 The anterior arm is bent at an angle of 45°; its 

 two portions are nearly equal, and the tip is 

 prolonged anteriorly and posteriorly into a T 

 shape. The muscles are attached to the ends 

 of the V and are six in number in each half of 

 the segment, two extending anteriorly and four posteriorly. The 

 two anterior muscles start from the outer anterior corner of the fifth 

 segment and run diagonally backward and inward side by side. The 

 outer shorter one (b) connects with the anterior horn of the T at the 

 end of the anterior arm of the V. The inner and longer one (a) con- 

 nects with the tip of the posterior arm of the V. Of the four pos- 



FiG. 12. — Muscles con- 

 trolling OPENING OF 

 OVIDUCT IN ERGASILUS 

 VERSICOLOR, a AND C, 



Closing muscles; 6 and 



d, OPENING MUSCLES. 



