THE OUTER TISSUES OF ANIMALS. 303 



dermal appendages originally serving the purpose of cloth- 

 ing, but afterwards differentiated into sense-organs. 



That eyes are essentially dermal structures seems scarcely 

 conceivable. Yet an examination of their rudimentary types, 

 and of their genesis in creatures that have them well deve- 

 loped, shows us that they really arise by successive modifica- 

 tions of the double layer composing the integument. They 

 make their first appearance among the simpler animals as 

 specks of pigment, covered by portions of epidermis slightly 

 convex and a little more transparent than that around it. 

 Here their fundamental community of structure with the 

 skin is easy to trace ; and the formation of them by differen- 

 tiation of it presents no difficulty. Not so far 

 in advance of these as much to obscure the relationship, are 

 the eyes which the Crustaceans possess. In every fish- 

 monger's shop we may see that the eyes of a Lobster are 

 carried on pedicles ; and when the Lobster casts its shell, the 

 outer coat of each eye, being continuous with the epidermis 

 of its pedicle, is thrown off along with the rest of the exo- 

 skeleton. This pedicle, which gives the name of "stalk- 

 eyed " Crustacea to a large group, is, strange as it may seem, 

 a transformed limb. Otherwise shown by the homologies of 

 the parts, this truth is made manifest by those transitional cases 

 in which the original form of the limb is retained, and the 

 transparent portion which serves as a visual organ is merely 

 a prominent patch on its under surface, somewhat like a blister, 

 spreading a little up the sides of the limb an arrangement 

 almost thrusting upon us the suspicion that an eye is a 

 modified portion of the skin. That which the outer appear- 

 ance suggests is proved by the structure within. Beneath 

 the transparent epidermic layer, there exists a group of eyes 

 of the kind which we see in an insect ; and these, according 

 to a high authority, are inclosed in the dermal system. De- 

 scribing the arrangement of the parts, M. Milne Edwards 

 writes: "But the most remarkable circumstance id, tha. the 

 large cavity within which the whole of these parallel 

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