PHYLUM ARTHROPODA 



235 



exceptionally, as a free-swimming stage in the Malacostraca, 

 the Nauplius stage in that sub-class being usually passed 

 through in the egg. The Nauplius 'is an oval unsegmented 

 body with a median eye, and three pairs of short appen- 

 dages provided terminally with long hairs. 



FIG. 136. Lepas anatifera. A, the entire animal; B, its anatomy, a', antennule; 

 c, carina; cd, cement gland; /, digestive gland; m, adductor muscle; od, ovi- 

 duct; ov, ovary; /, (in B) penis and (in A) peduncle; s, scutum; t, tergum and 

 testis; vd, vas deferens. (From Lang's Comparative Anatomy, after Darwin 

 and Claus.) 



.Most of the Entomostraca are free-swimming, and the 

 majority of them, such as the water-fleas (Fig. 135) and 

 their allies, are of almost microscopic minuteness, though a 

 few, such as Apus and the brine-shrimp, are of compara- 

 tively large size. Many Entomostraca, however, become 



