362 



a position which these structures will naturally take on 

 sinking to the bottom. 



The young Limulus on hatching is a trilobiti-form, free 

 swimming, commonly mero-planktonie organism, without 

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



Gigantostraca. — These I !rus1 acea are entirely extinct, being 

 represented by Eurypterus and Pterygotus. They were 

 undoubtedly marine, and probably belonged to both nekton 

 and vagrant benthos. 



Trilobitae. — The trilobites are extinct Palaeozoic 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 distribution 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 

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

 ber of fossil trilobites we can not judge the number of indi- 

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

 may represent the cast-off exoskeletons of one individual. 



Decapoda.—The decapods belong chiefly to the benthos, 

 inhabiting either fresh or salt water, rarely the land. 

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

 They commonlv 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 exceeding fifty fathoms. The range of indi- 

 vidual species is often great, AJpheus avarus, for example, 

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

 2,500 fathoms. Among the hermit crabs occur some forms 

 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 -'500 meters. They sometimes 

 inhabit the shells of land snails and have been observed 

 climbing trees. Among the true crabs, or I'raehyura. 

 shallow-water species predominate, comparatively few 



