PIGMENTATION OF THE EYES OF MOLLUSCS 6j 



seen to consist of radiating groups of cells, resembling the ommatidia of 

 compound eyes of insects and Crustacea. The cells are differentiated into 

 sensory cells and pigment cells, the former, in relation with clear 

 refractile elements, ending distally in a conical inclusion of vase-shaped 

 form, similar to the " crystal-cone " of certain composite eyes, e.g. the 

 paired eyes of Apus. The eyes lie in close relation to nerve fibres and 

 ganglia, and although a direct connection between the sensory cells and 

 the nerve fibres has not been traced with certainty, the eyes have been 

 shown experimentally to be very sensitive to light. Thus the shadow of 

 a hand held over an aquarium containing the Tubicola will cause instant 

 retraction of the filaments. These eyes have been found in other species, 

 e.g. P. oculifera, and it is thought that they may be of some interest in 

 interpreting the mode of development of the eyes of arthropods. 



The Pigment in the Eyes of Insects, Arachnids, and Crustacea 



The deposit of pigment in the form of small, dark-brown granules of 

 approximately uniform size in the hypoderm or epithelial cells and of 

 coarse granules in branched mesenchyme cells is very like that found in 

 the eyes of vertebrates. Moreover, in the more highly differentiated eyes 

 of species belonging to these three classes, there is a differentiation into 

 separate sensory-cells and pigment-cells. There is also frequently a 

 more abundant deposit of pigment in the immediate neighbourhood of 

 the eyes, but absence or reduction of pigment in the region of the lens. 

 On the other hand, in the composite eyes each rhabdite or crystalline-rod 

 or crystalline-cone is usually surrounded and isolated from its fellows by 

 specialized pigment cells. Further, the pigment cells of the retina in 

 many species are of two types, namely, (i) epithelial, and (2) pigment- 

 bearing, " intrusive " mesenchyme cells, which are elliptical or multipolar 

 in form and are believed to have wandered into the retinal zone from the 

 exterior, as in the medial or central eyes of Euscorpius and Limulus 

 polyphemus (Fig. 95, Chap. 11, p. 133 and Fig. 87, p. 125). 



Pigmentation of the Eyes of Molluscs 



The same characteristic features are present with regard to the deposit 

 and distribution of pigment in the eyes of molluscs as in the arthropods 

 and vertebrates, namely (1) a uniform deposit of small, deep-brown 

 granules in the epithelial cells — including the hypoderm cells and inner 

 layer of columnar retinal cells ; (2) an irregular deposit of granules of 

 unequal size in branched or fusiform mesenchyme cells. The deposit 

 in the tall columnar cells of the retina is, moreover, usually confined to 

 the middle third of the cell, while the inner end of the cell which is turned 



