174* PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
a similar position. Possibly the latter is a series of muscles rather 
than a single bundle, but the sections are not clear on this point. 
The loop-enveloping muscles, which form the bulk of the flexors, 
arise in common with the ventralis muscles from the anterior 
boundary of the segment, but ascend as a broad sheet of fibers 
(pi. 10, fig. 59b). They soon become transverse in course and grad¬ 
ually separate into two parts, the loop or circularis and the envel¬ 
oping or oblique. The fibers composing the loop muscles, probably 
augmented by fibers from the metacleis insertion, turn longitudinally 
so that the anterior end of the muscle lies dorsad of the transversalis, 
meeting the posterior end of the loop muscle of segment one, while 
its posterior end passes into segment three and there meets the 
loop muscle of that segment above the transversalis. The middle 
portion of the muscle is depressed and the successive arcs are very 
characteristic in longitudinal sections. Toward the mid-plane of the 
body a slip (pi. 10, fig. 59e, x) passes down from the anterior end of 
the muscle to become attached at the union of segments three and 
four, but whether on the sternum of segment three or into the 
articular membrane cannot be determined. Still nearer the mid¬ 
plane, the whole muscle, the anterior fibers changing first, becomes 
a longitudinal descending muscle. 
The fibers which go to form the enveloping muscle retain a more 
generally transverse course as they are separated from the fibers of 
the loop muscle; and therefore this muscle lies across the belly of 
the loop muscle (pi. 10, fig. 59d). Toward the mid-plane of the body 
it becomes in its turn descending, and goes down closely associated 
with the descending part of the loop muscles (pi. 10, fig. 59f, a* 2 ). 
The attachments of the descending portions of these muscles are not 
clearly shown in the sections. At times they are apparently at the 
articulation between the segments three and four; again the muscles 
seem to pass on to the next posterior articulation, a discrepancy 
which is possibly due to an attachment in part at both points. The 
loop muscle is small in Cambarus and is largely or perhaps wholly 
derived from the metacleis insertion. In Homarus it is a large mus¬ 
cle and derived from the metacleis insertion and attached at its 
descending end to the sternum of the third segment. In Homarus 
the slip x is attached to the sternal plate of segment three. The 
enveloping muscle in both of these Macrura and in allied forms is 
very large. 
