THE PECTOSTRACA. 253 



The Ostracoda either attach their eggs to aquatic plants, 

 or carry them about between the valves of the carapace. 



Claus ' has worked out the development of Cypris, which 

 passes through nine successive stages, distinguished from one 

 another, not merely by the shape of the carapace, but by the 

 number and form of the limbs. An ecdysis of the chitinous 

 cuticle of the body and carapace terminates each stage of de- 

 velopment. When the Cypris leaves the egg, it resembles a 

 Nauplius, in possessing a single median eye and only three 

 pairs of limbs (the future antennules, antennas, and mandi- 

 bles) ; but none of these are divided into two branches. The 

 body is laterally compressed and has a bivalve carapace. 



The changes undergone by the marine Ostracoda after 

 they leave the egg are much less marked. 



Fossil Ostracoda abound in strata of all ages, from the 

 older palaeozoic formations onward ; and, so far as the char- 

 acters of the carapace furnish evidence, the most ancient 

 forms differed very little from those which now exist. 



The Pectostraca (Rhizocephala and Cirripedia) leave 

 the egg as a JVduplhis, provided with three pairs of limb-like 

 appendages, of which the anterior pair are simple, while the 

 two posterior pairs are bifurcated (Fig. 68, A). An addi- 

 tional pair of filiform appendages subsequently makes its ap- 

 pearance in front of the undivided pair of members, in most 

 cases ; and there is a discoidal carapace, the antero-lateral 

 angles of which usually become greatly produced. Subse- 

 quently, the carapace becomes bivalve (as in many JPhyllo- 

 poda, and in the Cladocera and Ostracoda), and the anterior 

 undivided pair of limbs are converted into relatively large, 

 jointed appendages, provided with a sucker-like organ. The 

 thorax grows and usually develops six pairs of appendages. 



Finally, the bivalve-shelled larva fixing itself by the 

 suckers of its anterior limbs, the prae-oral region of the head 

 becomes enlarged, and is converted into the base, or pe- 

 duncle, in ordinary Cirripedes ; while it gives off the root- 

 like processes which grow into the tissues of the animals on 

 which the Hhizocephala are parasitic. The Pectostraca are 

 almost all hermaphrodite, a condition which is very excep- 

 tional among Arthropods. They possess no heart. 



The Cirripedia. — It can hardly be a matter of reproach 



1 " Entwickelungsgeschichte von Cypris " (1868) ; and " Grundziige," 

 p. 487. 



