180 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.. 
muscles are already much degenerated, the commissure between the 
ganglia of the second and third segments shortens by one third, the 
next posterior commissure shortens to a less extent, and commis¬ 
sures 4 and 5 lengthen. These changes bring the ganglia into their 
definitive distances from each other, accompanying the changes in 
the lengths of the respective abdominal segments preparatory to the 
moult to the sixth stage. The displacement of the anterior ganglia 
to the left is one of the last changes before the glaucotlioe stage 
closes, and occurs as the columella prominence arises. 
The Shell in the Ontogeny. 
Rathke noted in 1840 that the developing hermit crab became 
slightly asymmetrical before the close of the zoea phase and all 
observers have recorded asymmetry for the chelipeds and usually 
also, dissimilarity for the uropods in the glaucotlioe phase. Agassiz 
(’ 75 ) went a step farther and found that a very considerable advance 
toward the adult asymmetry might be attained before the larva ever 
entered a shell. It seemed clear from his account that this could 
not be regarded as the invariable sequence; but how far it might be 
looked upon as typical remained uncertain. lie says of the change : 
“ When the moult has taken place which brings them to the stage 
when they need a shell [my sixth stage] we find an important change 
in the two pair of feet now changed to shorter feet capable of propel¬ 
ling the crab in and out of the shell; we find that all the abdominal 
appendages except those of the last joint are lost; but the great dis¬ 
tinction .... is the curling of the abdomen ; its rings .... are quite 
indistinct and the test covering it is reduced to a mere film. It is 
therefore natural that the young crab should seek shelter for this 
exposed portion of his body.” His figures, published in 1882 
(Faxon, ’ 82 ), corrected and furnished a partial interpretation of this 
text. But they seemed to indicate that these changes might be only 
a beginning of a gradual metamorphosis to the complete adult form. 
It has therefore remained for the present research to show that the 
adult structure, so far as its plan is concerned, is completely attained 
during the glaucotlioe stage, and that typically the metamorphosis 
is inaugurated before a shell is taken. Nevertheless the young 
crab always enters the shell before the changes are far advanced, 
