REPORT ON THE ISOPODA. 23 



refractive lens-like body or " phaosphere." The shape of these bodies as well as their 

 position varies considerably ; sometimes they are quite spherical, occasionally they are 

 oval, and more rarely bean-shaped ; they are more usually found in front of the nucleus 

 of the retinula cell, sometimes within its swollen anterior extremity ; often they lie behind 

 the nucleus, and in a few cases they were observed in close contact to the nucleus and 

 slightly to one side. The phaospheres of all the four cells of a single retinula in some 

 instances occupied an approximately similar position, though more generally this was not so. 



I only succeeded in making out these structures in SeroUs iKiradoxa ; in Serolis 

 schythei and Serolis com uta they appear to be absent. 



Serolis cornuta. — In this species the cells composing the retinula are longer and 

 more slender than in Serolis schythei or Serolis paradoxa, and the pigment upon their 

 outer surface is comparatively feebly developed, being chiefly massed round the rhabdom ; 

 in other respects their structure is similar, and very thin longitudinal sections show that 

 intrinsic pigment is present within the cells themselves in addition to the intrusive 

 pigmentiferous connective tissue corpuscles which clothe them externally. The rhabdom 

 is a remarkably complicated structure, and differs greatly from the simple rhabdom found 

 in the two species just described. It is displayed in figs. 3, 9-15, which have been 

 drawn from preparations teased out in glycerin after having been depigmented by means 

 of nitric acid and in figs. 7, 8, 16, 17, which rej)resent longitudinal sections of this 

 portion of the eye undepigmented. In the latter the rhabdom is seen to consist of a 

 number of coiled threads running in various directions and coated externally with a thick 

 layer of black pigment. The lower extremity of the rhabdom projects into the " hyaline 

 cells" and is surrounded by their substance. In fig. 4 is displayed a teased preparation 

 of the eye of Scrolls cornuta undepigmented, and it may be seen that the rhabdom ends 

 below in blunt rounded prolongations which project into the "hyaline cells"; the lower 

 extremity of each of these prolongations — which indicate the composition of the rhabdom 

 out of four rhabdomeres^is devoid of pigment, so that the rays of light can pass through. 

 In longitudinal sections of course only one or two of these can be seen, though there are 

 in reality four. 



The shape of the rhabdom can best be made out by teased depigmented preparations ; 

 fig. 3 represents a single retinula prepared in this way ; the rhabdom is here seen in 

 profile and resembles an irregular coil of rope projecting below into the hya^line cells 

 (//) and continued into a thin filamentous prolongation which appears to pass through the 

 substance of these cells or between them, and extends nearly as far as the lower end of 

 the retinula cells. Figs. 9-15 represent the rhabdom and the two hyaline cells viewed 

 in such a position as to bring out the symmetrical character of the former. The form of 

 the rhabdom, however, differs widely in detail; some of the varieties are displayed in 

 these figures ; in all it appears to consist of a median unpaired portion which is continu- 

 ous below with four symmetrically arranged portions which project into the substance 



