SENSORY ORGANS AND RECEPTION 309 



about the central axis: these are pigmented externally and differentiated 

 at the axial border into a hyaline rod (the rhabdome). Each ocellus also 

 contains a so-called eccentric receptor cell, distinguished by being bipolar 

 and non-pigmented. From both kinds of cells nerve fibres run proximally 

 into the optic nerve. The median eyes are lens-eyes and each consists of a 

 vesicular ocellus containing a large cuticular lens. Receptor elements in 

 the retina are organized as groups of sensory cells (retinulae). Each group 

 can be regarded as a distinct ocellus made up of primary sensory neurones 

 which give rise to optic nerve fibres (63). 



Compound Eyes. These are composite structures made up of many 

 distinct units, each of which acts as a directional photoreceptor. The units 

 are separated from one another in some degree by pigmented screens, 

 and possess fixed focusing devices (Figs. 8.6, 8.8). Each unit is stimulated 

 by light coming from some particular direction, and the entire lighted 

 area to which the eye is exposed is thus divided into a large number of 

 separate fields, corresponding to the number of receptor units. The total 

 image thus formed by a compound eye consists of a patchwork or mosaic 

 of light and dark, corresponding to intensity differences in the visual field. 



Compound eyes occur in some polychaetes, lamellibranchs and are 

 especially characteristic of arthropods. 



The typical compound eye of crustaceans is composed of units called 

 ommatidia, each of which consists of photoreceptors and dioptric struc- 

 tures. Externally the eye is divided into facets, corresponding in number to 

 the underlying ommatidia. The facets are biconvex cuticular plates which 

 in sum form the cornea of the eye. Underneath the facet is a crystalline 

 cone which serves as a lens and which is secreted by vitrellae or crystal cells. 

 Below the lens is the retinula which consists of a set of from four to eight 

 cells around a central refractive rod known as a rhabdome. The retinular 

 cells are the photosensitive elements and proximally they give rise to 

 nerve fibres which form an optic nerve and extend to the optic ganglion. 



Each ommatidium is demarcated from its neighbours by retinal pigments. 

 These consist of a proximal retinal pigment localized in the retinular cells, 

 and a distal retinal pigment contained in distal pigment cells which 

 surround the crystalline cone and part of the rhabdome. In addition 

 there is a reflecting substance contained in reflecting cells lying basally 

 between the ommatidia; when exposed this substance forms a reflecting 

 layer or tapetum. Pelagic species of decapod crustaceans show much 

 variation in the distribution and amounts of retinal pigments. Eyes of deep- 

 water species from the aphotic zone, such as Sergestes grandis and S. 

 tenuiremis, are rich in reflecting pigments. Others, such as S. arcticus and 

 Petalidium obesum from the photic zone, lack reflecting pigment but have 

 a large amount of proximal retinal (screening) pigment (37, 118, 156, 157). 



Camera Eyes. These eyes possess a lens capable of focusing an image 

 on the retina. The most highly organized camera eyes among invertebrates 

 are those of cephalopods. The eye of Nautilus is a simple cup-shaped 

 depression with an opening through the integument. In dibranchiate 



