PHYSIOLOGY OF THE INVERTEBRATA. 363 



In Adaciiii, Homarus^ and other Devaimda, it may be re- 

 marked that both the olfactory and auditory organs are 

 lodged in the antennnliu. 



The eyes of the CmAacca are formed on a plan very 

 similar to those of the Insccta. Sometimes they are simple ; 

 but generally they are compound eyes, and in all the higher 

 Ci'usUu'ca they are carried on movable peduncles, an arrange- 

 ment not met with in any of the other classes of the 

 Arfhrojioda. 



" The Cirripcdia, the Fenellina, and the Lrrnwodca alone 

 are without an organ of vision ; and even here this deficiency 

 occurs only during the last phases of their retrograde meta- 

 morphosis, when these animals remain fixed to foreign 

 bodies.* There is, moreover, in the other orders, here and 

 tliere a genus, which contains blind individuals : such is the 

 case with the females of certain parasitic Isopoda ; f and the 

 same remark applies to some subterranean Miiriapoda.'' + 



The eyes of Adaciis and other Dccapoda are two in number 

 — one seated at the extremity of each of the ophthalmic 

 peduncles, the cuticle of which is continuous with the 

 transparent cornea. The corneal membrane is divided into 

 numerous minute square facets, each of which corresponds 



* The adult Cirripedla, notwithstanding the absence of eyes, are very- 

 sensitive to light (Von Siebold), 



t Bopyrun, Jone, Phryxus (i.e., the 9). 



* Pohjiletmun, Cryptoi)'<, Gcophllun, and BlaniuUm. The blindness of 

 those and other animals is generally attributed to the effects of disuse. 

 Concerning the blind cave-crabs, Darwin in the Origin of Species (p. 1 10) 

 says : "In some of the crabs the foot-stalk for the eye remains, though the 



eye is gone As it is difficult to imagine that eyes, though useless, 



could be in any way injurious to animals living in darkness, their loss may 

 be attributed to disuse." On the other hand, Mr. W. P. Ball says: "The 

 cave-crabs which have lost their disused eyes, but not the dinuxed eye-xtaUm, 

 appear to illustrate the effects of natural selection rather than of disuse. 

 The loss of the exposed, sensitive, and wonse-than-useless eye, would be a 

 decided gain, while the disused eye- stalk, being no imrticular detriment to 

 the crab, would be but slightly affected by natural selection, though open 

 to the cumulative effects of disuse." (See Ball's book : The Effects of Use 

 and Disuse, p. 17.) 



