756 



REPOltT — 1884. 



II ' 



On the w6stern shores of South America, around the broken coast of Patagonia 

 it has been taken in the channels Jjotween the rocky islands that lie aloiiir the 

 shore; others have been taken amonpf the Philippine Islands; also frmu the 

 deeper water around New Guinea; two species oil" the Fiji Island.s from near tln' 

 New Hebrides; whilst their near ally and conjrener Po/i/dtcli'ii, which is re. 

 presented by foin- species, has been taken in the Mediterranean and olF the coast of 

 Spain, iiKmid- Atlantic and oif the Fiji Islands, as well as near Kermadec and Xew 

 Guinea, and the beautiful Pohjchele^ cnuifera, that bears so near a reseniblamo to 

 the extinct Hiyon of jjeological record, was captured in the West Indian Seas. 



These genera wonderfully show tliat where conditions remain unaltertMl how- 

 little a thinpc is time, even when measured by <reolo<rical icons, in the history of the 

 progressive growth of species. 



However different J'Jn/on may be from Poh/chelcg, the difference in striK'tiiral 

 evidence is not greater than that wlilcli 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 ion^ 

 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 s') far beyond the 

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

 Glyphocra7yfmij-'m which the eyes are remarkable for their large and well-developed 

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

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

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

 eyes are large and conspicuous organs. 



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

 only had the opportunity of observing it before it has quitted the i^^^;:; — 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 Alphans, in which the eyes are reduced in size and 

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

 it possesses organs of vision of considei'ablo size. The animals as they grow appear 

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

 one to believe tliat the adult 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 Avith very distinct surrounding 

 conditions. 



Instead of inhabiting the deeper recesses of the sea, the genus A.'^facus and it^ 

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

 larger islands. 



Species exist in the Northern and Southern Hemispheres. 

 Those of the northern genera differ in the number of the hranchia from tho-e 

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

 than those of North America, and tliose 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 ]<]astern America, altlioui^li 

 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 witli those of Australia, wliich again ditler 

 from the species of New Zealand ; while the solitary'- species in Madagascar diflevs 

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

 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. • 



