128 BULLETIN OF THE 



the preceding paragraphs has been spoken of as ancestral, it is not to be 

 supposed that the condition which it presents must be regarded us necessa- 

 rily its simplest form. I feel tolerably confident, however, that the prim- 

 itive ommatidium must have been at least as simple as I have assumed 

 it to be. Possibly its retinula may have been composed of less than five 

 cells, as is that seen in some Copepods ; although, as I have previously 

 remarked, the condition of the lateral eyes in these Crustaceans is 

 probably influenced by degeneration, and therefore may not represent a 

 primitive stage. What might be regarded, however, as a more primitive 

 form of ommatidium than that which I have described, may be seen 

 in the eye of the Chaetopod Nais (Carriere, '85, pp. 28, 29). In this 

 worm the eye lies in the hypodermis on the side of the head, and con- 

 sists of a few relatively large transparent cells, the proximal faces of 

 which are in part covered by pigment cells. It is probable that the 

 transparent cells are merely dioptric in function, and that the pigment 

 cells are nervous. The transparent cells may therefore be looked upon 

 as the forerunners of cone cells, and the pigment cells at their bases as 

 retinular cells not yet differentiated into a retinula. It is not difficult 

 to imagine the origin of an ommatidium from a single one of the trans- 

 parent cells and its accompanying pigment cells, and, by an increase in 

 the number of such groups, the production of a retina like that of the 

 compound eye of Arthropods. 



This view of the orig'n of the ommatidia in Arthropods is irreconcila- 

 ble with that recently advanced by Watase ('90), according to whom 

 each ommatidium is to be regarded as a pit formed by an involution of 

 the hypodermis. The supposed cavity of this pit occupies nearly the 

 whole length of the axial portion of the ommatidium, and is filled by 

 the secretions of the cells constituting its wall. The secretion in the 

 deeper part of the pit forms the rhabdome ; that which is produced 

 nearer its mouth, the cone. During the formation of the pit, the hypo- 

 dermal cells are believed to retain such mutual relations that their moi*- 

 phologically distal ends lie next its cavity ; hence the secretions produced 

 by these ends, the rhabdome and cone, are to be regarded as modifica- 

 tions of the chitinous cuticula of the outer surface of the body. 



Ingenious as this theory is, I have not been able to convince myself of 

 its tenability. It may be urged against the assumption that the retinu- 

 lar cells occupy a proximal position and the cone cells a distal one on the 

 wall of a hypodermal pocket, that in Gammarus the retinular cells extend 

 from the distal to the proximal face of the retina, and that in Homarus 

 the cone cells have a corresponding extent ; these conditions show that 



