VI COMPOUND EYES 147 
cells” (7). 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 
(f) called the 
“membrana fenes- 
trata.” Each om- 
matidium is iso- 
lated from its 
fellows which sur- 
round it by a 
complete cylinder Fie. 108.—A, Sections (diagrammatic) of Crustacean com- 
of pigment, part pound eye, A, with pigment in light-position for mosaic 
5 § : vision; B, with pigment in dark-position for refractive 
of which is especli- vision. ¢, Corneal lens ; c.g, corneagen cells ; cr, erystal- 
ally erowded round line cone; /, basal membrane, or membrana fenestrata ; 
N ip, irido-pigment ; 2, nerve; 7, retinula; rh, rhabdom ; 
the crystalline rp, retino-pigment ; v, vitrella. 
cone, and is known 
as “irido-pigment” (7p), while the part which surrounds the 
rhabdom is called “ retino-pigment ” (7p). 
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 hght 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 
