MUSEUM OF COMPARATIVE ZOOLOGY. 85 



innermost. These differences, although at first sight somewhat con- 

 tradictory, are not matters of great importance, for it is probahle that 

 each time an Isopod sheds its cuticula and a new one is formed, the 

 lens assumes, at successive stages of its growth, outlines which coincide 

 very closely with those recorded by the different observers. Thus, an 

 early stage would be represented by the concavo-convex lens described 

 by Leydig, an intermediate stage by the plano-convex lens figured by 

 Sars, and the final condition by the biconvex lens mentioned by Miiller. 

 Either this is the explanation of the differences, or the observations of 

 Leydig and Sars are probably erroneous, for the results of the more 

 recent investigations point to the conclusion that the facets in Isopods 

 have the form of a biconvex lens. Facets of this shape have been seen 

 by Grenacher (77, p. 29) in Porcellio, and by Eellonci ('81 a , p. 98) in 

 Sphseroma. According to my own observations, they also occur in 

 Idotea, Asellus, Porcellio, and, as I shall show subsequently, in Serolis. 

 In the four genera mentioned the inner face of each facet is distinctly 

 convex; this is also true of the outer face in Asellus and Porcellio. In 

 Senilis and Idotea (Plate V. Fig. 50), however, the outer face is so 

 slightly curved that it is difficult to decide whether its curvature is that 

 of the general corneal cuticula or one peculiar to the facet itself. 



That the cone in Isopods is composed of two segments was first ob- 

 served by Leydig ('64 a , p. 41, and '64, Taf. VI. Fig. 8) in Oniscus. Ac- 

 cording to this author, each segment is spherical. Each ommatidium, 

 therefore, contains two spheres, and these, as Leydig's figure shows, are 

 placed side by side immediately below the corneal facet. 



It is now well known that in many Tsopods, especially in the wood- 

 lice, the cone itself is nearly spherical, and its two segments would con- 

 sequently be hemispheres, not spheres as figured by Leydig. How Ley- 

 dig's statement of the spherical shape of the segments can be accounted 

 for, is not apparent. Since the two spheres described by him occupy 

 the same relative positions as the hemispherical segments of a normal 

 cone, there is not much question in my mind that they represent these 

 segments. Possibly their separation and spherical form may have been 

 due to the swelling action of some reagent which Leydig may have used 

 to make the tissue transparent. A cone composed of two segments has 

 been observed by Sars ('67, p. 110) in Asellus, by Leydig ('78, p. 256) 

 in Ligidium, by Grenacher ('77, p. 29) in Porcellio, by Bellonci ('81 a , 

 p. 98) in Sphseroma, by Sye ('87, p. 23) in Ja?ra, and by Beddard 

 ('90, p. 368) in Arcturus. In the three genera which I have examined, 

 Idotea, Asellus, and Sphseroma, each cone consists of two segments. 



