co.v/'orxn eyes of AirriinoroDs. ;?oi 



out how an oimniitidinm like that of Limulutt may ho converted 

 into one of a more complex type. 



a. Gena-al Sketch of the Coiapound Eijr. 



Tlie compound eye of Zimulits is placed in the dorso-lateral 

 angle of the prosomatic shield. In the fully grown animal the 

 outline of the eye is bean-shaped, with its longer axis parallel to 

 the longitudinal axis of the body. This bean-sliaped area is 

 slightly protuberant from the surrounding level ; the chitinous 

 covering of the body is thinner, softer, and more translucent in 

 the region of the eye than on the rest of the dorsal surface. 

 Through the translucent chitin we see a large number of circular 

 spots, all of a uniform size. On further examination we find 

 that each spot corresponds to a conical projection of the chitin, 

 which fits into a cavity in the skin. This conical thickening of 

 the chitin constitutes the lens or lens-cone, and the group of cells 

 forming the walls and bottom of the open cavity constitute the 

 essential part of the ommatidium. 



The cells which form the walls of the pit are elongated and 

 highly pigmented, forming the perineural cells {p. n, Fig. 6, 

 PI. XXIX) ; those cells which form the bottom of the pit 

 undergo extreme enlargement compared with the perineural 

 cells, and elongate along the longitudinal axis of the pit. They 

 are grouped in a characteristic manner, reminding one of the 

 arrangement of cells in the taste-bulb of a vertebrate. This 

 bulb-like group of cells is the essential part of the retina. In 

 my preliminary account of the subject* I called this group of 

 cells an ommatidium. Properly speaking, the term ommatidium 

 includes the whole group of cells forming the walls and bottom 

 of the pit, together with the lens-cone and the thi(-k chitinous 

 covering corresponding to the area occupied by a single pit. In 

 this latter sense the term ommatidium will be used in the 

 present paper, and the bulb-like group of cells at the bottom of 

 the pit will be designated as the sensory part of the ommati- 

 dium. 



In the sensory part of the ommatidium, which consists of two 

 portions, a central a and a peripheral b, the arrangement of the 

 cells is quite regular. 



' loc. cit. p. 70. 



