202 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
and the absence of a distinguishable nerve center for the first abdo- 
* 
minal segment are more adult than larval. On the other hand, 
the anatomy and positions of the livers, and lateral caeca, the mus¬ 
culature of the stomach, the lack of an intestinal caecum, and the 
great elongation of the achitinous portion of the gut, the position of 
the sexual cells beneath the pericardial septum, and the preponder¬ 
ance of ganglion cells over fiber masses in the nervous system, are 
essentially as in the zoeae. Also many organs may be interpreted 
as presenting palingenetic as well as developmental characters. The 
cephalic position of the green glands, the thoracic position of the 
livers, the structure of the abdominal muscles and arteries, the com¬ 
plete set of pleopods, and the segmented abdomen may possibly 
serve as examples of this. 
The metamorphosis by which the structures attain the adult 
type of arrangement, commences before a shell is taken, although 
the larva usually enters a shell within the first forty-eight hours 
after the moult from the zoea phase. Through it, the livers, sexual 
cells and green glands become wholly or in part abdominal, the 
arterial system in this region is reorganized, and its muscles and 
pleopods degenerate. The anatomy becomes completely adult in 
type before the glaucothoe period ends. 
The stimulus of a shell is not necessary for the completion of this 
metamorphosis, any more than for its inauguration. The glaucothoe 
that has never entered a shell attains the adult structural type 
exactly like the glaucothoe which may have taken a shell immediately 
after the moult from the zoea phase. The shell is, nevertheless, very 
important. The length of the glaucothoe period bears a direct rela¬ 
tion to the time that elapses between the moult from the zoea phase 
and the entrance into a shell. Under normal conditions the period 
ranges from four to eight days, lasting five days for a majority of 
individuals; but deferring or preventing the taking of a covering 
for the body prolongs it, so that it may reach six or seven days for 
a majority of individuals. The range, however, is not altered. 
This effect is derived from the presence or absence of the shell, and 
is not dependent, except indirectly, on any peculiarities of its form. 
The shell also deeply affects the health of both glaucothoe and 
adult. Among glaucothoe which do not obtain a covering for the 
body or only obtain one after a delay of some days, the death rate 
is higher than among more normally reared individuals. The form 
