48 Traces of Unity in the Appendicular 



a cornea. The moveable ocular peduncle is very long, 

 much longer than in the prawn, in the podophthalmic 

 crabs, in the Gelasimus telescopicus more especially, 

 where it projects considerably beyond the edge of the 

 carapace. Here, indeed, the term podophthalmic is 

 rightly used, for the eye is as much a foot as an eye : 

 and still more so in the Ocypode cerophthalmus, where 

 the peduncle, in addition to being long and jointed at 

 its base, is prolonged as a calcareous spine far beyond 

 the part to which the compound eye is attached, and 

 where, consequently, the organ may be rightly looked 

 upon as half-eye, half-antenna. The close relationship 

 of the eye to the antenna is also seen in the gasteropods, 

 where two of the " horns " carry small simple eyes, one 

 on each, and two are plain antennae where, in fact, the 

 " horns " agree in being inversible or eversible like the 

 finger of a glove, or rather like the throat of a bryozoic 

 polype, and disagree only in this, that two have eyes, 

 and two are eyeless. Nor is this podophthalmic arrange- 

 ment peculiar to the crustaceans or gasteropods. On 

 the contrary, each of the two eyes of the nautilus and 

 of many other cephalopods is supported upon a short 

 peduncle containing a cartilage which is evidently the 

 homologue of that which is met with in the same place 

 in the pedunculated eyes of the sharks and rays. In 

 many of the cephalopods, also, the eye presents a pecu- 

 liarity which leads to the same conclusion by another 

 way. At all events, I find it difficult to look at this 

 form of eye without thinking of the sucker on the arms, 

 and of the polypodal or radiate type of development 

 which finds expression in both. For what is the actual 

 case ? It is that of an eye the cornea of which is pierced 

 near its centre by an opening through which the sea- 

 water enters and bathes the front of the crystalline 



