ARTHROPODA 263 



larva is usually retained, but some forms have in addition a 

 compound eye. 



The Ostracoda are dioecious, and the males are easily dis- 

 tinguishable from the females ; the former usually possess some 

 organs for holding the latter ; in GypriAina this is found on 

 the second antennae, in Cypris on the second maxillae, and they 



Fia. 156. — Transverse sectibn through the hody 

 6 and shell of Cyp-ris Candida. After Zenker. 



""—-4 ^- ^'^^^'• 



2. Inner fold of inantle lining shell. 



1 3. Ligament between the two halves of the 



shell. 



4. Adductor muscle. 



5. Body. 

 2''^\1 6. Intestine. 



7. Liver tubes extending into the shell. 



8. Testes sending prolongations into the shell. 



are also provided with more highly developed sense organs. 

 The ovaries and the testes may, like the Uver diverticula, 

 extend into the lining of the shell (Fig. 156). The accessory 

 male organs are very complicated, and the spermatozoa attain 

 a great size, being in some cases longer than the body. Cyjpris 

 attaches its ova to water-plants, but Cypridina carries them 

 about within the shell of the female until they hatch out. 



Order 3. COPEPODA. 



Charaotekistics. — EntoTnostraca with an doTigated segmented 

 hody, without a dorsal shell. The thorax tears four or five 

 biramous swimming feet ; the abdomen, which consists of four 

 segments, is devoid of limbs. Some species are parasitic, and 

 are then more or less degenerate. 



An enormous number of very variously modiiied Crustacea 

 are included in the group Copepoda. The free-swimming 

 forms are distinguished by the constant number, and by the per- 

 sistence, of their paired appendages. The parasitic forms, which 

 are very numerous, undergo every stage of degeneration, and in 

 the extreme cases lose aU trace of their Entomostracan affinities 

 in the adult condition. The systematic position of these forms 

 is, however, clearly shown by the history of their developement. 



