Elytra of the Aphroditacean Annelids. 239 
annelid in a double row. Each elytron is attached to a pe- 
duncle or “scale-tubercle,” the surface of attachment, of 
eircular or oval form, being situated about the centre of the 
elytron, and the attachment being effected through the inter- 
mediation of a series of muscular fibres, by whose contractions 
the various movements of the scales are brought about. The 
elytra are usually attached to every alternate segment, the 
intermediate segments having as their equivalents the cylin- 
drical dorsal cirri. In structure and mode of development 
the elytra and the cirri may be said to be essentially identical. 
Hach consists of an infolding of the integument enclosing a 
nerve, the infolding in the one case taking a cylindrical 
form, while in the other it becomes compressed and scale-like. 
Such a cutaneous fold ought to contain representatives of the 
cuticular, the subcuticular, and the muscular layers of the 
integument ; and such we find to be actually the case. Hach 
scale contains three principal tissues, viz. (1) an investing 
cuticle, (2) a double layer of cells or cell-equivalents, and (3) 
a fibrous layer. 
(1) The cuticle varies considerably as regards its degree of 
development. Sometimes, as in Aphrodita and Hermione, it 
is exceedingly delicate, and develops no appendages ; in other 
cases, as in Jphione and many species of Lepidonotus and allied 
genera, its upper layer attains a considerable thickness and 
density, and may be variously sculptured on the upper surface; 
where appendages are present, such as fimbrize or bristles, it 
forms an investment for them when it does not constitute their 
entire substance. 
(2) The cell-elements, representing the subcuticular layer 
of the general integument, take the form of a complete layer 
of polygonal squames lying immediately beneath the cuticle. 
This layer is sometimes transparent, the outlines of the cells 
being only discernible with difficulty ; at other times the cells 
are charged wiith pigment-granules, a lighter space in the 
centre being apparently the expression of a nucleus. ‘This 
double layer of cells was first pointed out by Ehlers* in 
Polynoé pellucida. I have found it inmost of the species which 
I have examined, though in some cases it does not appear to 
be distinct, and sometimes (phione) the upper layer becomes 
chitinized. 
(3) In focusing deeper than the upper layer of cells, Ehlers 
states that he could distinguish a series of dots, which he 
represents as arranged in radiating lines, and which he regards 
as indicating the existence of some tissue between the two 
layers in the “ cavity” of the scale. 
* Die Borstenwiirmer, p. 109 (1864). 
