318 PHYSIOLOGICAL DEVELOPMENT. 



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. 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. Describing the arrangement of the 

 parts, M. Milne Edwards writes : " But the most remarkable 

 circumstance is, that the large cavity within which the whole 

 of these parallel columns, every one of which is itself a per- 

 fect eye, are contained, is closed posteriorly by a membrane, 

 which appears to be neither more nor less than the middle 

 tegumentary membrane, pierced for the passage of the optic 

 nerve; so that the ocular chamber at large results from 

 the separation at a point of the two external layers of the 

 general envelope." Thus too is it, in the main, 

 even with the highly-developed eyes of the Vertebrata. 

 " The three pairs of sensory organs appertaining to the 

 higher senses/' says Prof. Huxley " the nasal sacs, the eyes, 

 and the ears arise as simple ccecal involutions of the ex- 

 ternal integument of the head of the embryo. That such 

 is the case, so far as the olfactory sacs are concerned, is 

 obvious, and it is not difficult to observe that the lens and 

 the anterior chamber of the eye are produced in a perfectly 

 similar manner. It is not so easy to see that the labyrinth 

 of the ear arises in this way, as the sac resulting from the 

 involution of the integument is small, and remains open but 

 a very short time. But I have so frequently verified 

 Huschke's and Remak's statement that it does so arise, that 



