230 ARTHROPODS. 



tribute greatly to the struggle for existence in that densely 

 crowded region. The lower Crustaceans are abundantly 

 represented in fresh-water, in pools, streams, and lakes. No 

 small proportion of the lacustrine fauna, which is often very 

 uniform over wide areas, consists of Crustaceans, which occur 

 both at the surface and at the bottom. Then there are a 

 few terrestrial Crustaceans, such as wood-lice, land-crabs. 

 A few forms, usually blind, are found in caves. 



History. — Fossil Crustaceans are found in Cambrian strata, 

 but the highest forms (Decapoda) were not firmly established 

 till the Tertiary period. Some of the genera, e.g., the 

 Branchiopod Estheria, living from Devonian ages till now, 

 are remarkably persistent and successful. How the class 

 arose, we do not know : it is probable that types like Nebalia 

 give us trustworthy hints as to the ancestors of the higher 

 Crustaceans ; it is likely that the Phyllopods, e.g., Apus, 

 bear a similar relation to the whole series ; the Copepods 

 also retain some primitive characteristics ; but it is difficult 

 apart from mere guessing to say anything definite as to the 

 more remote ancestry. The frequent occurrence of a Nauplius 

 larva might lead one at first thought to suppose that the 

 ancestral Crustacean must have been like a Nauplius. It is 

 likely, however, that the Nauplius is a larval reversion to a 

 much simpler form than that of the ancestral Crustacean, 

 much in the same way as the Trochosphere larva of Mol- 

 luscs is doubtless very much more primitive than the first 

 Mollusc. We naturally think of a segmented worm-type as 

 a plausible starting-point for Crustaceans, and it is not diffi- 

 cult to understand how a development of cuticular chitin 

 would tend to produce a flexibly jointed limb out of an 

 unjointed parapodium, how the mouth might be shunted a 

 little backwards, and two appendages and ganglia a little for- 

 wards, and how division of labour would result in the differen- 

 tiation of distinct regions. 



Economic Importance. — Many of the Decapods are very 

 good to eat ; many smaller Crustaceans feed the fishes. A 

 large number which devour debris are useful cleansers. 

 Some are the hosts of important parasites, e.g., that Cyclops, in 

 which the guinea worm i^Dracunculus or Filaria medinensis) 

 passes part of its life. 



