BIONOMIC CHARACTERS OF CRUSTACEA 1027 



shield, frontal sensory organs and three pairs of limbs. After 

 a series of moults the Cypris stage is reached, in which the larva 

 is enclosed in a bivalve shell, like that of the Ostracoda. During 

 these stages the larva belongs to the niero-plankton. When it 

 settles down and becomes attached it passes through a pupa stage, 

 during which the transformation of the larva into the cirriped 

 takes place. 



Phvllocarida. These are mostly extinct forms, represented in 

 all the divisions of the Pakeozoic. Several living genera ( Nebalia, 

 etc.) are regarded as belonging here, and these are marine. They 

 are swimmers, and their development is direct, without metamor- 

 phosis. Some of the Palaeozoic forms referred to this class were 

 undoubtedly marine ( Stenotheca, Ribeiria), but the majority seem 

 to have been fresh water forms living in the rivers of the Palaeozoic 

 lands. This is inferred from their peculiar occurrence in sediments 

 which could only have been formed at the mouths of streams and 

 in playa-like basins. At any rate, they did not seem to be normal 

 inliabitants of the open sea, but, if marine at all, lived near the 

 mouths of great streams. 



Schizopoda, Stouiatopoda, Sympoda. These are marine Crus- 

 tacea, capable of swimming about by the use of their abdominal 

 legs and caudal fin. Larval stages are often wanting. 



Dccapoda. The decapods belong chiefly to the benthos, inhab- 

 iting either fresh or salt water, rarely the land. Pelagic species also 

 occur, some of which are good swimmers ; while a few belong to 

 the plankton and others to the epi-plankton, living on the Sargas- 

 sum. They commonly feed on living or dead animal matter. The 

 bathymetric distribution varies greatly, though the majority of 

 species are confined to comparatively shallow water, generally not 

 exceeding fifty fathoms. A considerable number, nevertheless, are 

 abyssal. The range of individual species is often great; Alpheiis 

 avarus, for example, is said to range on the Australian coast from 

 less than ten to about 2,500 fathoms (Challenger), though this is 

 questioned by Ortmann (21:75). 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 mountains 

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

 shells of land snails and have been seen climbing trees. Among 

 the true crabs, or Brachyura, shallow-water species predominate, 

 comparatively few occurring below 400 fathoms. A number of 

 species live in fresh water or on land. The majority of decapods 

 leave the egg in the zoea stage, in which the abdominal region is 

 perfectly segmented, though still without appendages, except per- 



