88 BULLETIN OF THE 



pointed star, a ray corresponding to a rhabdomere. Each ray projects 

 into its retinular cell, not between two cells. My own observations on 

 Porcellio confirm Grenadier's statements. A second representative of 

 this type of rhabdome has been described by Bellonci ('81, p. 98) for 

 Sphseroma. Here, however, the rays, although they agree in number 

 with the retinular cells, project between the cells, not into tbem. 



The second type of rhabdome is well represented in the eye of Arc- 

 turns furcatus. In this species, according to Beddard ('90, pp. 3G8, 

 3G9), 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, rhb.) 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 h>st. 



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 Grenadier ('79, p. 107), and it is probable 

 that the pigment cells described by Bellonci ('81, p. 99) as intervening 

 between the retinular cells in Sphseroma 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, vt. 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. Grenadier believed that those in Porcellio are undifferentiated 

 hypodermal cells ; this interpretation probably holds good for those in 

 Sphseroma and Idotea. 



The hyaline cells, the second kind of accessory cells, have been iden- 

 tified by Beddard ('87, p. 235, '88, PI. XXX. Fig. 9, A.) in Mga. 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. 



