THE SIMPLE EYE 



151 



the surface into a cup of pigmented cells, and the conducting prolongations of 

 the cells are turned towards the direction of the incident light, have the con- 

 figuration of an " inverted "' retina. In a sense, also, the composite simple eyes 

 of Chsetognaths and some of the smaller Crustaceans to be discussed immediately 

 are also of this type. 



AGGREGATE EYES 



The AGGREGATE EYE is a Suitable name to designate an accumu- 

 lation of ocelli so closely packed that they bear a superficial resemblance 

 to a compound eye although each is anatomically separate. Such an 

 arrangement is seen in its most simple form in starfishes (Plate I), in 

 such insects as the male Stylops ^ or in Myriapods (Fig. 210),^ in which 

 it appears as a cluster of ocelli. 



Stylops 



Figs. 128 and 129. The Aggregate Eye of Braxchiomma yEsicctoscM. 



Fig. 



128. — Cross-section through a branchial fila 

 ment of the worm. 



BV, blood vessel ; C, cuticle 

 F, fibril ; L, lens ; X, nucleus ; 

 visual cell (after Hesse). 



Fig. 129. — Axial section 

 through two ocelli. 



Car, cartilage ; Cil, cilia ; Ep, epidermis ; 

 ON, optic nerve ; P, pigment cells ; R, 



An entirely difTerent type of aggregate eye is seen in the branchial 

 filaments of some sedentary polychsete worms and in certain lamelli- 

 branch molluscs wherein the organ has a superficial structural resem- 

 blance to a compound eye but each element contains only one sensory 

 cell (Figs. 175-6). In the first case, the eye of the polychsete, 

 Branchiomma vesiculosum, is made ujd of a spherical group of elements 

 resembling ommatidia, but since each contains only a single cell it 

 should be considered an ocellus and the eye is technically a simple 

 organ of the aggregate type (Brunotte. 1888 ; Hesse, 1896-99) 

 (Figs. 128 and 129). It is to be remembered, however, that in such 

 tube-worms these structures do not seem to be essential for the animal's 

 characteristic response to changes in light intensity (Millott, 1957). 

 A similar arrangement is seen in the eyes of the lamellibranch molluscs. 

 Area and Pechmcidus (Carriere, 1885 ; Patten, 1886 ; Hesse, 1900). 



Branchiomma 



221, 



' P- 



110. 



