88 BULLETIN OF THE 
pointed star, a ray corresponding to a rhabdomere. Lach ray projects 
into its retinular cell, not between two cells. My own observations on 
Porcellio confirm Grenacher’s statements. A second representative of 
this type of rhabdome has been described by Bellonci (’81, p. 98) for 
Spheroma. Here, however, the rays, although they agree in number 
with the retinular cells, project between the cells, not into them. “ 
The second type of rhabdome is well represented in the eye of Are- 
turus fureatus. In this species, according to Beddard (’90, pp. 368, 
369), the distal portion of the rhabdome, although surrounded by six 
retinular cells, is bounded by four perpendicular sides. Each of the six 
cells appears from its position to contribute to the formation of the 
rhabdome, and yet in the greater part of this structure segments cor- 
responding to rhabdomeres are not visible. In its proximal portion, 
however, the rhabdome, according to Beddard, is divided into six rhab- 
domeres, each of which is applied to its proper retinular cell. In Idotea 
robusta the rhabdome (Plate V. Fig. 48, rid.) is nearly square in trans- 
verse section. So far as I have been able to discover, it does not show 
at its proximal end any indication of rhabdomeres. 
Of these two types of rhabdome, the one in which the rhabdomeres 
are evident is probably more primitive than the one in which their in- 
dividuality is almost, if not completely lost. 
The retinas of Isopods may contain, in addition to those already 
mentioned, two other kinds of cells. Of these the one most frequently 
met with fills the space between ommatidia. Cells of this kind have 
been identified in Porcellio by Grenacher (’79, p. 107), and it is probable 
that,the pigment cells described by Bellonci (’81, p. 99) as intervening 
between the retinular cells in Sphzroma belong to this class. I have 
observed interommatidial cells in Idotea; here they contain few or no 
pigment granules, but are easily recognized by means of their nuclei 
(Plate V. Fig. 54, nl. h’drm.). 
The source of these cells is not definitely known, but there appears to 
be no evidence in favor of their having been derived from outside the 
retina. Grenacher believed that those in Porcellio are undifferentiated 
hypodermal cells ; this interpretation probably holds good for those in 
Sphzeroma and Idotea. 
The hyaline cells, the second kind of accessory cells, have been iden- 
tified by Beddard (’87, p. 235, ’88, Pl. XXX. Fig. 9, 2.) in Aga and 
Cirolana. Since these cells are best developed in the eyes of Serolis, a 
full description of their structure will be deferred until the account of 
the eyes in that genus is given. 
