128 



HA RD WICKE'S SCIENCE- G OSS IE. 



the ends'of which rest on an accumulation of pigment. 

 These form elementary eyes. 



The water-flea {Daphnia pulcx) is possessed of two 

 eyes,?one simple and the other compound, or multi- 

 lenticular. The former is of the simplest construction, 

 consisting merely of a small mass of pigment behind 

 a not very highly refractive body : it is represented 

 at or. 



The compound eye, though of simple optical con- 

 struction, 'is a much more complex organ, and shows 



the compound eye of the water-flea is medial and 

 single. This is also noticed in Cyclops, Cypris, and 

 some others. 



The illustration was drawn with a camera-lucida 

 from a living specimen. It is magnified about three 

 hundred diameters, and seen in optical vertical 

 section. 



The carapace, modified to form a cornea, is seen 

 at c a. The space c. s. corresponds in position with 

 the aqueous humour of the vertebrate eye, but striic- 



■L 



Fig. 63. — Eyes of Water-Flea. X -500. 



in a marked degree the way Nature adapts herself to 

 circumstances. 



The water-flea is transparent, hence there is no 

 need for the eye to be protruded or stalked, as in the 

 higher crustaceans. The transparent carapace, or 

 shell, forms a protection for the eye, whilst not in- 

 terfering with its visual powers. Again, from the 

 transparency of the animal, there is no need for the 

 eye to be divided into separate halves, as in the higher 

 members of the same family. Accordingly we find 



turally with the conjunctival sac, the carapace ca 

 being only a functional cornea. This space is lined 

 with a delicate membrane, which is seen to be 

 doubled on itself at the upper and lower angles ; the 

 redundancy at this part is to allow of the eye's being 

 moved with freedom. 



The eye itself is an ovoid mass of pigment, repre- 

 sented at pg. It is surrounded on all sides, except 

 at a portion of the posterior surface, with a number 

 of highly refractive spherical lenses {c. /.). Advantage 



