MUSEUM OF COMPARATIVE ZOOLOGY. 117 



barus, Palinurus, Pagurus, Cancer, and Cardisoma. In the majority of 

 these, a fibrillar axis could be distinguished. 1 In Cambarus, as in Homa- 

 rus, the nerve fibrillar spread over the distal portion of the rhabdome. 



In Serolis an exceptionally interesting condition is presented. At the 

 level of the basement membrane each retinular cell contains a large fibril- 

 lar axis (Plate VI. Fig. 64, ax. n.). This becomes somewhat subdivided 

 in the more distal portion of the cell, and in the region of the retinular 

 nucleus it is represented by a cluster of several smaller axes (Fig. 63). 

 At the level of the hyaline cell, these however cannot be distinguished 

 (Fig. 62), but the scattered condition of the pigment granules in this 

 plane is probably to be accounted for by the presence of many separate 

 fibrils in the substance of the cell. In the region of the rhabdome an 

 immense number of fine lines can be seen extending from the retinular 

 cell into the substance of each rhabdomere (Fig. 61). These, I believe, 

 represent the fibrils of the nervous axis. They have been previously 

 observed in Serolis by Watase ('90, p. 291), and are so readily visible 

 that there can be no question as to their presence. Each fibril is per- 

 pendicular to the longitudinal axis of the ommatidium, and extends 

 through the rhabdomere to its axial surface. Before reaching this, 

 however, the fibril passes through what seems to be a delicate mem- 

 brane. When closely examined, this membrane often has the appearance 

 of a row of dots instead of a line, and in several cases I have been unable 

 to discover any traces of it. What its significance is, I am at a loss to 

 say. As I have previously observed, when the elements of the retinula 

 are separated the rhabdomere shows no tendency to break along this line. 

 Since the structure is pierced by the fibrils, and does not appear to be 

 a natural plane of rupture, and since sometimes it is apparently absent, 

 I believe it may be considered, from a morphological standpoint at least, 

 as a secondary and rather unimportant modification within the rhabdo- 

 mere itself. 



If I am correct in maintaining that the nerve fibrils in Serolis terminate 

 in the rhabdomere, it is probable that they have a similar method of 

 ending in all other Crustaceans, and in such instances as Homarus, 

 where they have been traced only to the surface of the rhabdome, their 

 actual termination has probably not been seen. 



1 A definite fibrillar axis was traced from below the basement membrane to the 

 region of the rhabdome in Gammarus (Plate I. Figs. 6-S), Porcellio (Plate V. Fig. 

 46), Idotea (Plate V. Figs. 53 and 55-57), Mysis (Plate VII. Figs. 87-80), Gono- 

 dactylus (Plate VIII. Figs. 101, 102), Palsemonetes (Plate IX. Figs. 116, 117), Cam- 

 barus, Pagurus, Cancer (Plate X. Figs. 130 and 131), and Cardisoma. 



