COMPOUND EYES 



147 



cells " (r). The rhabdom is a transversely striated rod, constituting 

 the true sensory part of each ommatidium, and is in connexion at 

 its lower end with 

 a nerve -fibre (n), 

 passing to the 

 optic ganglion. 

 The rhabdoms rest 

 upon a membrane 

 (/) called the 

 " membrana fenes- 

 trata." Each om- 

 matidium is iso- 

 lated from its 

 fellows which sur- 

 round it by a 

 complete cylinder 

 of pigment, part 

 of which is especi- 

 ally crowded round 

 the crystalline 

 cone, and is known 

 as " irido-pigmenfc 

 rhabdom is called 



B 



Fig. 103. — A, Sections (diagrammatic) of Crustacean com- 

 pouud eye, A, with pigment in light-position for mosaic 

 vision ; B, "with pigment in darlc-position for refractive 

 vision, c, Corneal lens ; eg, corneagen cells ; cr, crystal- 

 line cone ; /', basal membrane, or membrana I'encstrata ; 

 ip, irido-pigment ; n, nerve ; r, retinula ; rk, rhabdom ; 

 rp, retino-pigment ; v, vitrella. 



(ip), while the part which surrounds the 

 ■ retino-pigment " (rp). 

 When the pigment is arranged in this way, as in Fig. A, 

 only those rays of light which strike an ommatidium approxi- 

 mately at right angles to the corneal surface can be perceived, 

 since only these can reach the top of the rhabdom ; the others 

 pass through the crystalline cones obliquely, and are absorbed by 

 the cylinder of pigment surrounding each ommatidium, so that 

 they neither reach the rhabdom of the ommatidium which 

 they originally entered, nor can they penetrate to the rhabdom 

 of neighbouring ommatidia. This gives rise to what is known 

 as " mosaic vision," that is to say, each ommatidium only 

 perceives the rays of light which are parallel to its long axis, 

 and in this way an image is built up of which the various 

 points are perceived side by side by means of separate eye- 

 elements. The distinctness and efficiency of this mode of vision 

 depends chiefly upon the number of ommatidia present, and the 

 completeness with which they are isolated from one another by 

 the pigment. Now this form of vision, depending as it does 



