756 report— 1884. 



On the western shores of South America, around the broken coast of Patagonia, 

 it has been taken in the channels between the rocky islands that lie along the 

 shore ; others have been taken among the Philippine Islands ; also from the 

 deeper water around New Guinea ; two species off the Fiji Islands from near the 

 New Hebrides ; whilst their near ally and congener Polycheles, which is re- 

 presented by four species, has been taken in the Mediterranean and off the coast of 

 Spain, in mid- Atlantic and off the Fiji Islands, as well as near Kermadec and New 

 Guinea, and the beautiful Polycheles micifera, that bears so near a resemblance to 

 the extinct Eryon of geological record, was captured in the West Indian Seas. 



These genera wonderfully show that where conditions remain unaltered how 

 little a thing is time, even when measured by geological reons, in the history of the 

 progressive growth of species. 



However different Eryon may be from Polycheles, the difference in structural 

 evidence is not greater than that which exists among species of the same genera 

 in recent epochs. 



These animals are inhabitants of the deep sea. Their seeing powers are re- 

 duced in construction and extent, and the organs are so hidden that they were long 

 supposed not to exist ; in the fossil representative they have not yet been determined. 



It has generally been supposed that this depreciation in the power of vision in 

 animals that live in deep waters is largely due to their being so far beyond the 

 reach of light ; but this can scarcely be the cause, inasmuch as that the genus 

 Glyphocranyon, in which the eyes are remarkable for their large and well-developed 

 condition, is found at equally great depths and frequently associated with them, 

 and, taking the several ranges at which species have been found, the average depth 

 of the WillemEesian group is less than that of other deep-sea forms in which the 

 eyes are large and conspicuous organs. 



An examination of the animal while yet in an embryonic condition — and I have 

 only had the opportunity of observing it before it has quitted the egg — shows that 

 in its earliest stages of development the young has organs of vision similar to the 

 ordinary crustacean type. 



Thus it is like the genus Alphaus, in which the eyes are reduced in size and 

 hidden beneath the carapace in the adult, while in the younger stages of existence 

 it possesses organs of vision of considerable size. The animals as they grow appear 

 to retrograde in the condition of their organs of sight, a circumstance that induces 

 one to believe that the adidt animals cease to exist under the same conditions as 

 the young, and depart from roaming in the open water to the hidden recesses 

 where exposed organs of vision would be useless and liable to injury. 



The consideration of these deep-sea forms gradually leads us to that of another 

 genus of no very distant structural character, but with very distinct surrounding 

 conditions. 



Instead of inhabiting the deeper recesses of the sea, the genus Astacus and its 

 allies dwell in the shallow fresh-water streams and lakes of the continents and 

 larger islands. 



Species exist in the Northern and Southern Hemispheres. 



Those of the northern genera differ in the number of the branchia from those 

 of the southern. The crayfish of Europe aud Asia possess fewer branchial plumes 

 than those of North America, and those of North America fewer than those of the 

 Southern Hemisphere. 



The species of Eastern Europe and Western Asia differ in general form from 

 those of Western Europe, although they correspond in their branchial arrangement, 

 while those of Western Europe closely resemble those of Eastern America, although 

 they differ from them in their branchial condition. 



The species of South America differ in general external form from those of the 

 Northern Continent, but correspond with those of Australia, which again differ 

 from the species of New Zealand ; while the solitary species in Madagascar differs 

 from all others structurally, but corresponds externally with that of Australia. 

 And the recently discovered Astacus from the Eocene strata of North America 

 possesses the same congenital features peculiar to the recent form of the Northern 

 Hemisphere. 



