THOMPSON: METAMORPHOSES ,OF HERMIT CRAB. 
187 
Effect of the shell on structure .— Since the shell determines to 
some extent the duration of the glaucothoe phase and also affects 
the health of the crab, a further influence might be perhaps expected. 
But in vain. The order in which the different organs develop from 
the larval to the adult type is the same whether the glaucothoe 
obtains its shell early or late in the period. It is independent of the 
presence or absence of the shell, and is not affected by its shape. 
Moreover, no difference in the structure or arrangement of the 
parts of the body of sixth-stage crabs reared under the various 
conditions can be detected, except in the reduction of the pleopods. 
A crab of this age, typically, has no pleopods on the right side of 
the body, but this is not invariably the case even among “ normally 
reared ” larvae, and some crabs retain rudiments of one or more of 
these appendages. The metamorphosis of the pleopods has been 
expressed by the following formulae (see page 158) : — 
Glaucothoe. 
Sixth Stage. 
Adult $. 
Adult 9. 
Segment 1 
0 : 0 
0 : 0 
O 
O 
0 : 0 
“ 2 
R : R 
Ru : 0 
0 : 0 
R : 0 
“ 3 
R : R 
R : 0 
R : 0 
R : 0 
“ 4 
R : R 
R : 0 
R : 0 
R : 0 
“ 5 
R : R 
R : 0 
R : 0 
R : 0 
“ 6 
+R : R— 
1 
+ 
+R : R— 
+R : R— 
However, since segments one and six do not vary, the formulae may 
be written in briefer form : glaucothoe = R, R, R, R : R, R, R, R ; 
sixth stage — Ru, R, R, R : O, O, O, O ; adult $ — O, R, R, R : 
O, . 0 , O, O ; and adult 9 = R, R, R, R : O, O, O, O. 
The nature of the variations from the normal sixth-stage formula 
under all conditions was studied in about five hundred crabs 
which had been reared from the glaucothoe as follows : 107 in dex- 
tral shells, 62 in straight shells, 103 in sinistral shells, and 221 with¬ 
out shells. Of these 493 crabs, 159 or thirty-two percent varied 
from the usual pleopod formula. The different variations were 
numerous, but they fall naturally into three groups : (a) those 
where one or more of the right appendages were retained as 
large (Ru + ). medium (Ru), or small (Ru —) rudiments, in order 
from before backward; (b) those where there was loss in number or 
size of the pleopods on the left side; and (c) those where there was 
