17l> PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
1. The transversalis muscles begin to lose their fibers at the 
time when the livers shift to the abdomen. 
2. After the shift is completed and the nephrosac has passed to 
the abdomen, the loop muscle’s fibers have become straight, and the 
descending portions of this and of the enveloping muscles are weak. 
The descending, lateralis, and ventralis muscles remain distinct, but 
the transversalis, the pleopodal muscles, and the intrinsic muscles 
of the pleopods themselves have disappeared. 
3. Then the integumentary muscles appear; the descending and 
the ventralis still show fibers, but only a few fibers can be found 
in the loop-enveloping muscles. The columella prominence is 
developing. 
4. Still later, traces of the enveloping and descending muscles 
can still be identified, but only with difficulty. The ventralis and 
integumentary muscles are somewhat more distinct. 
The flexor muscles of the adult hermit crab then, evidently lack 
the transversalis elements, and retain only remnants either of 
descending or of lateralis muscles. The relative proportions of the 
remaining flexors, the ventralis and the loop-enveloping muscles, 
are not readily or surely determinable. Probably the ventralis 
plays the larger role. 
The thin layer of integumentary muscles seems to be derived from 
scattered fibers that lie in the same position during the glaucothoe 
stage. These may also give rise to the apical fibers of the muscles 
in the columella prominence but the basal fibers of this organ are 
certainly derived from the ventralis of the third segment and it is 
possible that the others are also. 
The theory that the flexor muscles of the hermit crab are a 
crowded series of “chevron” muscles is scarcely tenable in face of 
this evidence from the study of their structure and metamorphosis. 
The support that it has received from the chevron-like muscles of 
Gebia and Callianassa also fails when a closer examination is made. 
In Gebia certainly, the “chevrons”* resolve themselves into loop- 
enveloping systems with a weak loop and insignificant transversalis, 
so that the preponderance of the enveloping element gives a notably 
oblique course to the muscles. 
The larval hermit crab has, therefore, abdominal muscles more 
like those of generalized Macrurous Crustacea, than those of such 
Thalassinids as Gebia, and it would be interesting in this connection 
