PHYLUM ARTHROPODA 



233 



their allies, are of almost microscopic minuteness, though 

 a few, such as Apus (Fig. 1 24) and the Brine-shrimp, are 

 of comparatively large size. Many Entomostraca, however, 



FIG. 125. Two stages in the development of Apus. A, Nauplms just hatched ; 

 B, 2nd larval stage ;fs. frontal sensory organ ; / 4, cephalic appendages ; I. 

 VII body segments and appendages. (From Lang's Comparative Anatomy. 

 After Claus.) 



become fixed in the adult condition as external parasites, 

 mainly of fishes. Many of these parasitic Entomostraca 

 undergo a degradation of structure, a retrograde metamor- 



