THOMPSON: METAMORPHOSES OF HERMIT CRAB. 
173 
of the functions which ordinarily belong to a superior abdominal 
artery. 
The abdominal musculature of the young Eupagurus was studied 
chiefly from serial sections. Although only longitudinal sections 
proved of value, this does not at all effect the accuracy of the results, 
since in examining the muscles of adult Cambarus or Homarus by 
means of sections, only those cut in the longitudinal plane are 
interpretable. Such sections, however, throw a great deal of light 
on the arrangement of the muscles in these Crustacea. And I feel 
the more confidence in my conclusions with regard to musculature 
in these forms from the fact that they are based on both dissections 
and sections, although they differ somewhat from those usually 
accepted. In any event, the abdominal muscles of the young 
Eupagurus are essentially like the muscles of Cambarus, Homarus, 
and other Macrura; and sections of each are mutually comparable 
(pi. 10, figs. 59, 60, 61). 
The muscles of Eupagurus reach their highest efficiency with the 
glaucothoe stage. At this time the extensors are well developed 
with a generally longitudinal course (pi. 10, fig. 59, ext ); the 
pleopodal muscles converge from a region of attachment above the 
hinge or metacleis where the segments interlock and are independent 
of the flexors (pi. 10, fig. 59a, plm). The flexors comprise several 
muscles, the descending, transverse, longitudinalis, and loop-envelop¬ 
ing. The arrangement of these various muscles can best be under- 
stood from the study of a single segment, selecting segment two as 
typical. 
The abdomen of the glaucothoe is highly convex and hence the 
attachment above the hinge, the metacleis insertion, is more dor¬ 
sal than in the flatter segment of Homarus. The descending and 
transverse muscles arise from this point. The former is a broad 
band whose ventral end is inserted at the articulation with segment 
one; the latter is cylindrical and runs first ventrally and then trans¬ 
versely to come into intimate union with its mate from the other 
side of the body. In Cambarus and Homarus the transversalis is 
flat and in the latter genus it arises as part of the loop-enveloping 
muscles. 
The lateralis or lateral longitudinal passes from the anterior to 
the posterior borders of the segment at the sides ; and ventrally 
above the nerve cord the ventralis or ventral longitudinal occupies 
