362 



a position which these strnctnres will natiirallY take on 

 sinking- to the bottom. 



The young Linmhis on hatching is a trilobiri-forni. free 

 swimming, commonly mero-phmktonic organism, withont 

 a candal spine. (J. S. Kingsley. ) 



Gignntostraca. — These Crustacea are entirely extinct, being- 

 represented by Enrypterns and Pterygotus. They were 

 nndonbtedly mai-ine, and probably belonged to both nekton 

 and vagrant benthos. 



TrilohitiV. — The trilobites are extinct Paheozoic Crustacea 

 of an undoubted marine habitat, probably able to swim as 

 well as crawl, and so belonging at one time to the nekton, at 

 another to the vagrant benthos. Whether or not a mero- 

 planktonic larva existed is not known, but this might be 

 assumed from the wide distriljution of some species. As trilo- 

 bites cast off their exoskeleton like Limulus, some of these 

 may have been floated for some distances, coming to lodge 

 wdiere trilobites never lived. It is certain that from the num- 

 ber of fossil ti-ilobites AA-e can not judge the number of indi- 

 viduals existing at a given place, since a number of sjtecimens 

 may represent the cast-off exoskeletons of one individual. 



Decnpuda. — The decapods belong chiefly to the l)enthos, 

 inhabiting either fresh or salt water, rarely the land. 

 Pelagic species also occur, some of which are good swimmers. 

 They commonly feed on living or dead animal matter. The 

 bathy metric distribution varies greatly, though the major- 

 ity of species are confined to comparatively shallow water, 

 generally not exceechng fifty fathoms. The range of indi- 

 vidual species is often great, Alpheus ararus, for example, 

 ranging on the Australian coast, from less than ten to about 

 2,500 fathoms. Among the hermit crabs occur some foi-ms 

 which have left their native element, and have taken to the 

 land. The "Challenger "" found some of them in the moun- 

 tains of the Antilles, up to 300 meters. They sometimes 

 inhabit the shells of land snails and have been observed 

 climbing trees. Among the true crabs, or Brachyura, 

 shallow-water species prech^minate, comparati\ely few 



