CEUSTACEA 



107 



every exuviation. The ommas are surrounded by pigment 

 cells designed to absorb or reflect the rays. The black or 

 dark brown absorptive pigment varies with the intensity of 

 light. It is not yet quite clear as to how the image or images 

 are built up by the eye as the instrument intervening between 

 the brain and the external objects. The ommatidia are so 

 disposed that they cannot all see the same object at once, 



A 



Corn 



Pigment'^ 



Crystalline cone- 



Pigment 



Rhabdome 

 enclosed in retinula" 



Basement membrane -=j 

 Nerve fibre p$ 



Cerebral ganglion 



, Visceral nerve 

 -Oesophagus 

 -Oes. commissure 

 -Sub-oes. ganglion 

 -3rd maxpd. ganglion 



-1st pereiopod ganglion 



-Aperture for sternal artery 



= = 5th pereiopod ganglion 

 -1st pleopod ganglion 



-6th pleopod ganglion 



FIG. 51. Nephrops. A, a single element, ommatidium of the compound eye ; 

 B, nervous system. After G. H. Parker. 



and an all-round picture of a wide range is more or less perfectly 

 conveyed to the brain by the two eyes. They have been 

 compared, or the assemblage of rhabdomes, to the rods 

 and cones of the vertebrate eye, and it may be imagined 

 that the picture is built up by each omma contributing as 

 much as lies in its way. If perfectly fresh eyes be held in a 

 favourable light, it will be found that the outer layer of pigment 

 does not prevent one seeing through the layer of crystalline 

 cones. In such a case the single reflected images of external 

 objects may be seen within the eye, occupying, according to 

 distance, more or less of the inner part of the layer of crystalline 

 cones. If the eye be placed under the microscope, and the 



