TYPE CRUSTACEA. 



381 



paired eye is present iii the larval stages of probably all 

 Crustacea, arid persists iu a more or less perfect form in the 

 adults of most Eutomostraca, a group which contains the 

 more primitive forms, and has even been detected in those 

 of some of the higher forms (e.g. Crangori). It consists when 

 well developed of three patches of pigment, forming cups in 

 each of which lies a group of clear cells from which nerve- 

 tibres arise passing to the optic nerve. 



The lateral eyes are composed of a number of units each 

 of which possesses all the parts of a visual organ and is 

 termed an ommatidium, and consequent- 

 ly these eyes have been regarded as an 

 aggregation of a number of individual 

 eyes, whence the term compound usually 

 applied to them. Each ommatidium is 

 a complicated structure consisting of 

 several parts (Fig. 171). The outermost 

 layer of each is a transparent cornea 

 which is continuous with the general 

 cuticle of the body, and in some forms 

 is only distinguished from this by its 

 transparency. More frequently, how- 

 ever, this cuticle becomes more or less 

 perfectly divided into a series of corneas 

 of an hexagonal or tetragonal shape, 

 one corresponding to each ommatidium, 

 the surface of the eye thus acquiring 

 a faceted appearance. Below the cuti- 

 cle 



FIG. 171. DIAGRAM OF 

 CRUSTACEAN OMMATI- 

 DIUM. 



C = cone-cell, 

 come the hypodermal cells (Off) Cff = coraeal hypodermis. 



which secrete it, arranged irregularly ^= crystalline cone. 



.*?... / DR = distal retinula. 

 without reference to the ommatidia m PR = proximal relinula 



the simpler non-faceted eyes, but in Rh .= rhabdom. 

 the faceted eyes with two hypodermal cells lying beneath 

 each cornea and constituting the corneal hypodermis. 

 Below these come the cone-cells (6'), two to four in number 

 as a rule ; these are elongated cells a portion of whose pro- 

 toplasm becomes converted into a refractive translucent body, 

 the crystalline cone ( CC), composed of as many segments as 

 there are cone-cells taking part in its formation, and sur- 



