160 



of a series of cells which on their outer side receive and 

 concentrate the rays of light. These are transmitted to 

 the retinulae by means of the transparent vitrellae. The 

 retinulae are connected with the nerve fibres, and aro, 

 therefore, the important part of the eye. Each 

 ommatidium is surrounded by a layer of pigment, so that 

 the sensitive retinulae are situated at the bottom of a 

 tube, which is completely separated from the tubes of the 

 other ommatidia because of the presence of the pigment. 

 Therefore, the retinulae of an ommatidium can only 

 receive light through the small cornea! facet at the distal 

 extremity of the particular ommatidium. 



The cornea (en.) is composed of three layers. On the 

 outside is a thin structureless layer the cuticle (cut.). 

 Beneath this is an outer pigmented layer (pig. I.), to which 

 the colour of the cornea is due. Below the pigment layer 

 is the deeper layer. Both inner layers exhibit longi- 

 tudinal striations. 



According to Watase, the corneagen, vitrellae and 

 retinulae are all modified epidermal cells, which, in the 

 case of the vitrellae and retinulae, have grown inward. 

 The cornea is a chitinous secretion of the corneagen. The 

 crystalline cone is primarily a chitinous secretion of the 

 vitrellae, and the rhabdome is a chitinous rod secreted by 

 the cells of the retinulae. 



The otocyst. 



The otocyst is found in the basal segment of the 

 antennule. It is a sac lined with a layer of chitin, which 

 is continuous with the outer chitinous integument. In 

 the megalopa stage and in the young crabs the otocyst is 

 open to the exterior, and in these stages otoliths are 

 present in the sac. In large crabs, however, there are no 

 otoliths, and the sac is completely closed except 



