THE OSTRACODA 67 



the vasa deferentia are probably always united by a transverse 

 duct shortly after leaving the testes. They are often of great 

 length and are coiled in a very complex fashion, differing in the 

 different groups. The distal part forms a ductus ejaculatmius of 

 complicated structure, with muscular walls. The paired penes are 

 traversed by the vasa deferentia, the terminal portion forming a 

 protrusible copulatory tube, and the penis is often armed with 

 chitinous hooks moved by special muscles. 



The spermatozoa are sometimes spherical (Asterope), more 

 commonly filiform. In the Cyprididae they are of a size which 

 relatively, if not absolutely, is unique in the animal kingdom. In 

 /'inifiii'i//!/'/* Hionstrosa the total length of the animal is *6 mm., while 

 the spermatozoa are 5'0-7'0 mm. long. 



DEVELOPMENT. 



The eggs are carried within the valves of the shell dorsally to 

 the body of the animal in the Cypridinidae, Cytherellidae, some 

 Cytheridae, and in the freshwater genus Darwinula, and in a 

 few cases the young are carried for some time after hatching. In 

 other cases the eggs are deposited on water-plants or shed free 

 in the water. The course of development is best known in the 

 Cyprididae. On hatching, the larva is already enclosed Avithin a 

 bivalve shell, but otherwise corresponds in structure to a nauplius 

 (Fig. 39). Three pairs of appendages are present, antennules, 

 antennae, arid mandibles, the last 

 two, however, not distinctly biramous. 

 In the Cytheridae the mandible at 

 this stage is rudimentary. The ss??\***&>^3^- -^ \ 



. . v , ij j fff. Swrt'C^^^-AL--' 



remaining limbs are added at suc- 

 cessive moults in regular order from 

 before backwards. According to j S 

 Miiller, it is not the case that, as 

 stated by Glaus, the fourth post-oral 

 limbs appear before the third. There 30 



is, however, a marked pause in the H wHrtofCtoHii ', antenna ; 



development before the appearance ", antenna; ad, adductor muscle ofcai ii- 

 .,,., , . J l , , . pace ; e, nauplius eye ; mil, mandible. 



or the third post-oral pair, and this, (Attcr ciaus.) 



as already pointed out, supports the 



view that a pair of appendages corresponding to the maxillae of 



other Crustacea is missing. 



REMARKS ON HABITS, ETC. 



The Ostracoda are abundant both in fresh waters and in the 

 sea, generally burrowing in mud or creeping among weeds. The 

 marine Halocypridae belong to the plankton. None are definitely 



