Elytra of the Aphroditacean Annelids. 241 
scale may be well seen in successfully-stained elytra of species 
of Polynoé; and there can be little room for doubt that these 
processes or papille are, in many instances at least, the 
end-organs of this elytral nerve. 
The functions with which the elytra may be supposed to 
be connected are (1) protection, (2) the production of phospho- 
rescent light, (3) sensation, and (4) incubation. 
(1) The protective function of the scales is in some cases 
the predominating one. Thus in Jphione the scales are of 
extreme density, and cover the entire dorsal surface with a 
complete armour, which the animal is incapable of throwing 
off, and which gives it a remarkable resemblance to a Chiton. 
In others the scales, though tough, are more readily detached, 
and in many instances do not completely cover the dorsal 
surface ; in many species of Polynoé again, they are so delicate, 
and are so readily parted with when the animal is irritated, 
that their direct protective action must be very slight ; while 
in genera such as Aphrodita, in which the dorsal surface is 
covered with matted hairs, the presence of elytra from this 
point of view seems unnecessary. 
(2) When certain species of Polynoé are irritated in the 
dark a flash of phosphorescent light runs along the scales, 
each being illuminated with a vividness which makes it shine 
out like a shield of light, a dark spot near the centre repre- 
senting the surface of attachment where the light-producing 
tissue would appear to be absent. The irritation communi- 
cates itself from segment to segment; and if the stimulus be 
sufficiently powerful, flashes of phosphorescence may run along 
the whole series of the elytra, one or more of which then be- 
come detached, the animal meanwhile moving away rapidly and 
leaving behind it the scale or scales still glowing with phospho- 
rescent light. Thespecies in which the phenomenon of phospho- 
rescence occurs are species characterized also by the rapidity 
of their movements, and also by the readiness with which the 
scales are parted with; and it seems not at all unlikely that 
the phosphorescence may have a protective action, the illumi- 
nated scales which are thrown off distracting the attention of 
an assailant in the dark recesses which the Polynoide usually 
frequent. 
(3) That the elytra act, like the dorsal cirri, as organs of 
some special sense seems probable from their abundant inner- 
vation, as well as from the presence, in many instances, of 
fimbrize and other appendages, some of which at least act as 
end-organs for the nerve-branches. These appendages, the 
form of which varies greatly, are processes of the upper wall 
of the scale, and probably consist of the cuticular, subcuti- 
