THOMPSON: METAMORPHOSES OF HERMIT CRAB. 177 
to know the type of musculature among the less specialized Tlialas- 
sinoids. This might throw some light on the extent to which the 
perfection of the Macruran type in the Eupagurus larvae is palin- 
genetic. For it is not possible to regard it as simply correlated with 
the active life of the zoea or glaucothoe. The very active megalops 
of the Brachyuran, Callinectes, although provided with enormous 
pleopodal and well developed descending muscles, has of the 
remaining possible flexors only the ventralis bundles; the metazoea 
of Pinnotheres and both the young zoea and the metazoea of Cancer 
show similar relations, with the addition of a few fibers which mav 
be doubtfully identified as loop-enveloping elements. Moreover, 
Gebia, like other Macrura, has in the larval stages a very perfect 
loop-enveloping system of flexors, and these are carried on into the 
first adolescent almost intact, although the animal is at this period 
quite inactive. 
The muscles of the stomach of the zoea and glaucothoe are very 
simple. At first only dorsal and ventral supporting muscles are 
present (pi. 9, fig. 47). But with the second stage two additional 
bundles arise with attachments near the point where the oeso¬ 
phageal plates will later appear and the third zoea adds two more 
which extend from the anterior face of the stomach forward to the 
cephalothoracic wall. This simple arrangement is retained by the 
metazoea .and by the glaucothoe until the very close of the latter 
period. Then it rapidly gives place to the complex musculature of 
the adult. The new muscles are developed from myoblasts which 
lie around and above the stomach from the third zoea onward. Of 
the zoeal muscles, the pair that extend from the region of the oeso¬ 
phageal plate are alone retained. 
It seems probable that the adductors of the mandibles in the 
adult are also new structures with the glaucothoe, and not deriva¬ 
tives of the fan of muscles that moves the jaws in the zoeae (pi. 9, 
fig. 49). But the moult from the fourth zoea produces so great a 
change in the plane of the mouth parts relative to the body axis that 
a satisfactory comparison between sections through this region in 
zoea and glaucothoe is impossible. The remaining muscles of the 
body call for no especial mention. The developing limbs remain 
filled with undifferentiated tissue until the fourth zoea and then 
fibers begin to make their appearance. 
The thoracic portion of the nervous system of the adult hermit 
