361 



regiinn. haviiio- been dredged by tlie " Challenger '" from nearly 

 3.000 fathoms. These abyssal cirripeds are usually at- 

 tached to nodules, dead or living shells, corals, lai'ge Crus- 

 tacea, to spines of sea-urchins and other objects.* The cirri- 

 peds. upon hatching fi-oni the egg, pass through several 

 larval stages, the first of which is the Nauplius stage. In 

 this the body is unsegmented with median frontal eye. 

 dorsal shield, frontal sensory organs, and three ]:)airs of 

 Hmbs. 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 

 mero-])lankton. When it settles down and becomes at- 

 tached, it passes through a pupa stage, during which the 

 transformation of the larva into the cirriped takes place. 



OstracodR.— The ostracods are marine or fresh-water 

 planktonic or vagrant benthonic Crustacea, whose imper- 

 fectly segmented body is enclosed in a l)i valve shell. The 

 majority of the marine forms are planktonic, living in 

 shallow water or moderate depths, though a fe\v si)ecies 

 were found by the "Challenger" at depths exceeding 2,000 

 fathoms. Some species are cosmopolitan, and the order is 

 repi-esented in nearly all waters. Their shells occur in nearly 

 all the bottom deposits. The animals are also abundant on 

 algce, over which they crawl slowly. The fresh-water 

 Cypris swims about, subject, however, to the influences to 

 which other members of the plankton are subject, or era wis 

 about over the vegetation. Cyjiris is also represented in 

 brackish and salt-water. The larva of Ostracoda is a 

 pelagic Xauplius. 



Xiphosurn. — The xii)h()surans are represented by the 

 single living genus Limulus, which is a marine vagrant 

 benthonic animal, though often swimming on the back 

 when young. The usual habitat of this animal is in shallow 

 water, where it is often partly l)uried in the nuid or sand. 

 Poi'tions of the coast are often strewn with the cast-off 

 exoskeletons of Limulus, which commonly lie on their back. 



*Agassiz, "88, U., p. 50. 



