EYES OF CRUSTACEANS III 



described as an upright compound eye, and since it consists of many 

 retinulas or ommatidia, arranged in a radiating manner side by side, it is 

 also a composite eye. It will be unnecessary to attempt a full description 

 of the detailed structure of the eyes of Astacus and Palcemon, as this has 

 been so admirably achieved by Grenacher in his great work on the Eyes 

 of Arthropods. It is worth noting, however, with reference to Fig. n, 



Fig. 73. 

 Compound eye of Caprella acutifrons, viewed from above and seen by transmitted 

 light under a low-power magnification of the microscope. The drawing 

 shows how light can pass upwards through the central axes of a particular 

 group of retinulae, towards the eye-piece of the microscope ; and it may be 

 presumed that in the natural condition the direction of light passing in the 

 reverse direction would be transmitted by definite groups of retinulae, whose 

 sensory cells would be those chiefly affected. By varying the position and 

 direction of the source of light, different groups of retinulce would be affected 

 and different fibres of the optic nerve would carry impulses to the brain. 

 (R. J. G.) 



that the smooth outer layer of the eye is continuous with the surrounding 

 cuticle and that the cells of the retina and its basement membrane are 

 directly continuous with the hypoderm cells and basement membrane 

 of the stalk ; also that the optic ganglia and fibres of the optic nerve are 

 enclosed in a loose, double-layered sheath, which will permit of free move- 

 ment of the stalk without injury to the optic nerve, and that this move- 

 ment can be brought about by the bands of striped muscle-fibre, some of 



