MUSEUM OF COMPARATIVE ZOOLOGY. 91 



cells, which occupy positions immediately under the cuticula and be- 

 tween the ommatidia. 



The facets in the corneal cuticula of Serolis, when viewed from the 

 exterior, are irregularly circular in outline, often approaching a six-sided 

 form. As I have already observed, they are arranged on the plan of the 

 hexagonal type. The distal face of each facet is flat, or only slightly 

 convex ; the proximal face is decidedly convex. The curvatures of the 

 two faces and the thickness of the cuticula in the facet of S. Schvthei 

 was about the same as that figured by Watase ('90, Plate XXIX. Fig. 1) 

 for the species which he studied. 



The cone, as Beddard ('84, p. 340) first demonstrated, and as "Watase 

 ('90, p. 290) afterwards confirmed, is composed of two nearly hemi- 

 spherical segments, which correspond to the two cone cells. The proto- 

 plasmic material of each cone cell covers the curved surface of the seg- 

 ment to which it belongs, and contains a nucleus in its distal portion. 

 These relations have been well shown by Watase ('90, Plate XXIX. 

 Fig. 1). 



From the condition presented even in advanced embryos (Fig. 65) 

 it is evident that the part of the cone earliest formed, is the one which 

 is nearest the applied faces of the two cone cells, and that from this as 

 a centre the cone has continued to increase outwards. Although at this 

 stage the outline of the cone itself is sharply marked (Fig. 65), the ex- 

 ternal limits of the cone cells are only approximately indicated by the 

 distribution of the pigment granules, which have begun to form in the 

 surrounding pigment cells. 



In Serolis, as in Porcellio and Idotea, the cone cells and the cells of 

 the corneal hypodermis are separated by the same perpendicular plane. 

 There are some complications in the structure of the cone cells which 

 can be discussed subsequently with greater clearness. 



The retinula in Serolis, as Beddard ('84, p. 340) first observed, is 

 peculiar in that it is composed of only four cells. My own observations 

 add almost nothing that is new to the previous accounts of this structure. 

 The figure which Watase has drawn ('90, Plate XXIX. Fig. 1) of the 

 characteristic form of the retinular cell when viewed from the side and 

 its relation to its rhabdomere, reproduces very closely the structural 

 conditions which I have observed in S. Schvthei. 



The rhabdome in Serolis has been carefully studied by Beddard ('88, 

 pp. 448-450). Owing to the complexity of its structure, one meets with 

 difficulties in attempting to interpret its parts in terms of the relatively 

 simple rhabdome of many Crustaceans. The peculiarities of this struc- 



