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 ahout the rhabdome, but in the region of 

 the nuclei of the other retinular cells, i. e. proximal to the basement 

 membrane. 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. 3(38), the retinula is also composed of six cells. 



In SphsBroma, Bellonci ('81, p. 98, Tav. II. Fig. 12) has figured and 

 described a retinula consisting of five 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 retinular of Isopods may be as few as five. My own observations, 

 however, do not. confirm Bellonci's account. In the species of Sphreroma 

 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 (Irenacher ('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- 



