PHYSIOLOGY OF THE INVERTEBRATA. 363 



In Astacus, Honiarus, and other Becwpoda, it may be re- 

 marked that both the olfactory and auditory organs are 

 lodged in the antennulse. 



The eyes of the Crustacea are formed on a plan very 

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

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

 Crustacea they are carried on movable peduncles, an arrange- 

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

 Arthropoda. 



"The Cirripedia, the Penellina, and the Lernwodea 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 

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

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

 same remark applies to some subterranean Myriapoda." % 



The eyes of Astacus and other Decapoda 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 Cirripedia, notwithstanding the absence of eyes, are very 

 sensitive to light (Von Siebold). 



i Bopyrus, .Tone, Phryxus (i.e., the 9). 



+ Polydesmus, Gryptops, Oeophilus, and Blaniulus, The blindness of 

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

 Concerning the blind cave-crabs, Darwin in the Origin of Species (p. no) 

 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 disused eye-stalhs, 

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

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

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

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

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

 and Disuse, p. 17.) 



