MUSEUM OF COMPARATIVE ZOOLOGY. 87 
it is usually somewhat smaller, and I therefore conclude that it is a 
rudimentary cell. It does not appear to contain a nucleus; granting, 
however, that it is a rudimentary retinular cell, one would look for its 
nucleus, not in the region about the rhabdome, but in the region of 
the nuclei of the other retinular cells, i. e. proximal to the basement 
membréne. Owing to the irregularity with which the fibrous ends of 
the retinular cells are arranged in this region, I have not been able 
to identify any nucleus with this rudimentary cell. Neither have I 
found any fibrous projections reaching from the rudiment of the cell 
toward the basement membrane such as might be expected provided 
the nucleus and a part of the rudimentary cell persisted below the 
membrane. Nevertheless, I believe, for the reasons already stated, 
that the retinula in Idotea robusta is composed of seven cells, one of 
which is extremely rudimentary. 
In Idotea irrorata (Plate V. Figs. 53, 55) the retinula consists of only 
six cells, all of which possess fibrillar axes, and are therefore probably func- 
tional as nervous structures. In one retina of the several pairs of eyes 
which I examined, there was a single ommatidium with seven functional 
cells (Fig. 54). With this one exception, however, I have not been able to 
find any trace of the seventh cell in Idotea irrorata. In Arcturus, accord- 
ing to Beddard (’90, p. 368), the retinula is also composed of six cells. 
In Spheroma, Bellonci (’81, p. 98, Tav. II. Fig. 12) has figured and 
described a retinula consisting of jive cells. These cells alternate with 
five other cells, which probably represent accessory pigment cells. If Bel- 
lonci’s statement is correct, it must be admitted that the number of cells 
in the retinule of Isopods may be as few as five. My own observations, 
however, do not confirm Bellonci’s account. In the species of Sphzroma 
which I have studied, there are seven cells in the retinula, four of which 
are large and three small (Plate V. Fig. 58). . All these cells pass through 
the basement membrane ; all the large ones, and certainly some of the 
small ones, are also connected with nerve fibres. 
These observations indicate that in the Isopods the retinula is com- 
posed of either six or seven cells. If Bellonci’s statements prove to be 
correct, this structure may be composed in some cases of only five cells, 
but my own observations are opposed to this view. 
The rhabdome in Isopods presents two types of structure, one of 
which has been well described by Grenacher (’77, p. 30) for Porcellio 
scaber. In this species the rhabdome is composed of seven rhabdomeres, 
each of which remains in connection with the retinular cell which pro- 
duced it. In transverse section the rhabdome has the form of a seven- 
