288 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
they shorten only to about five sevenths of their original length in- 
stead of to three sevenths. Thus their long retention in the dark 
seems to have prevented a return to the more completely contracted 
condition normal for the light. The fact that the more peripheral parts 
of the cells in the contracted condition are the more wrinkled indicates 
that the axial part has retained its contractile nature more completely 
than the peripheral parts, and suggests the idea that, since this axis 
contracts in a definite direction, it must possess something of the nature 
of a muscular core. It seems to me probable that, whilst the periphery 
of these cells may be characterized by amoeboid movements, the core 
acts in a more circumscribed way, much as a muscle would. If this is 
so, the distal and proximal migrations, as well as the expansion of the 
cell body, are probably manifestations of its amaboid movements, 
while its shortening is probably due in the main to the muscle-like con- 
traction of its central core. Objection might be raised to this com- 
bination of different modes of motion, were it not generally admitted 
that muscular action is, after all, only a more circumscribed form of 
amoeboid movement. 
The presence of a contractile axis in the distal retinular cells is further 
rendered probable by the fact that in Mysis (Parker, '91, p. 120) an 
axial core free from pigment has been observed in each distal retinular 
cell. At the time I first noticed these cores I suspected that they might 
be the remains of nervous axes, but I now believe there are stronger 
reasons for suspecting them of being contractile bodies. 
The following table gives by way of summary the periods required 
for the completion of the various photomechanical changes in the retina 
of Palemonetes. 
From dark to light. From light to dark. 
Proximalretinularcells . . . . . jhr.to $hr. 4 hr. to 1 hr. 
Distal retinular cells 1} hr. to 13 hr. 1} hr. to 2 hr. 
Accessory pigment cells 4 hr. to 1 hr. 14 hr. to 2 hr. 
It is a noteworthy fact, that of these changes those that take place 
in the light (positive stimulus) are always accomplished more rapidly 
than the corresponding reversals in the dark. To this statement an 
apparent exception may be found in the tables on pages 285 and 286, in 
which are recorded the periods for the contraction and expansion of the 
