CHAP. III.] OF SENSE ORGANS. 61 



serving to condense the light thereon ; and these media 

 in still other organisms are sufficiently like a lens to 

 he adequate to form a definite image of an external body 

 on the layer of pigment, which (on its other side) is in 

 contact with a nerve-expansion directly communicating 

 with a contiguous ganglion. Numerous simple structures 

 of this kind may exist apart from one another, as in many 

 Bivalve Mollusks : or they may be far more numerous and 

 closely aggregated, so as to form such compound-eyes as 

 are met with in Crustacea and in Insects. Or individual 

 ocelli may be perfected, as in Spiders or lower Crustacea, 

 and most notably of all among the Cuttlefish tribe, in 

 the representatives of which two moveable eyes are met 

 with whose organization is just as perfect as those of Fishes. 

 The difference in degree and range of sensitiveness 

 between the simple ' eye- specks ' of some of the lower 

 Worms, and the elaborate visual organs of the highest 

 Mollusks and Insects is enormous. The range and keen- 

 ness of sight also become progressively extended, so that 

 creatures with the more perfect eyes are capable of 

 appreciating impressions from objects more and more 

 distant, and the various actions which become established 

 in response to impressions habitually made upon such 

 sensitive surfaces also increase enormously in number, 

 variety, and complexity. The relation between the keen- 

 ness of the sense of sight and the great powers of loco- 

 motion possessed by Insects has long been recognized by 

 naturalists. Prof. Owen thus alludes to it: "The high 

 degree in which the power of discerning distant objects is 

 enjoyed by the flying insects corresponds with their great 

 power of traversing space. The few exceptional cases of 

 blind insects are all apterous, and often peculiar to the 

 female sex, as in the Glow-worm, Cochineal-insect, and 

 parasitic Stylops." 



