92 BULLETIN OF THE 



ture can be approached most satisfactorily perhaps from the side of its 

 adult anatomy. 



In a transverse section of the distal end of the rhabdome, five struc- 

 tures can be observed (Fig. 61). Four of these (Fig. 61, r/ib'm.) are 

 squarish pieces confluent on one side with a retinular cell, and in contact 

 with one another only at their angles The sides of these pieces which 

 are directed towards the axis of the ommatidium are convex, and to- 

 gether bound a central area which contains the fifth or axial structure 

 (cl. con.). Each of the squarish pieces also exhibits a line slightly concave 

 towards the axis of the ommatidium. This line, which might be taken 

 for the separation between the axial and peripheral structures, is in real- 

 ity entirely within the latter. That these are five separate structures 

 is indicated by the fact, that in transverse section, when for any reason 

 the elements have been broken apart, the separation almost always occurs 

 on the lines which 1 have described as the limits of the different pieces. 



Evidently the squarish masses (r/ib'm.) on the axial faces of the retinu- 

 lar cell correspond to the rhabdomeres of other Crustaceans, and like these 

 structures are produced by the cells to which they are attached. It is 

 more difficult to explain the axial element, for it shows no indication of 

 having been produced by the surrounding retinular cells, nor are there 

 other colls in the neighborhood to which its production could be 

 referred. 



"When the longitudinal extent of these structures is considered, the 

 difficulty of explaining the axial portion is increased. In S. Schythei 

 the rhabdomeres extend only a short distance distally and proximally, 

 but throughout the whole of that distance they are closely applied to 

 the axial face of the retinular cells. This condition has been well figured 

 by Watase ('90, Plate XXIX. Fig. 1), and supports the statement 

 alread\' made that these bodies correspond to the rhabdomeres in other 

 Crustaceans. I have never observed a rhabdomere, such as that figured 

 by Beddard ('87, p. 234), in which the proximal half of the structure 

 is not in contact with the retinular cell. The axial part has a much 

 more considerable extent in a longitudinal direction than the rhab- 

 domeres. Apparently it is continued proximally into a fibrous bundle 

 which stretches towards the basement membrane, where according to 

 Beddard ('88, p. 449) it may terminate as a single fibre. 



From what has just been stated it must be evident that the so called 

 rhabdome of Serolis consists of two sets of structures, one of which 

 includes the four rhabdomeres and the other the axial part with its prox- 

 imal fibrous prolongation. 



