88 



ZOOLOGY. 



abdominal segments arising after successive moults, until 

 tlie adult form is attained. 



Many Eiitomostraca are parasitic, living on the gills of 

 fishes, etc., and consequently undergo a retrograde devel- 

 opment, losing the jointed structure of the body, the ap- 

 pendages being more or less aborted, while the body in- 

 creases greatly in size. Such are the fish-lice, represented 



Fio lOG— Section through the thorax of Apus: en, 1-6, the sis endites; ex, ex- 

 opodal or respiratory portion of the hinb forming the flabellum, /6; c, cara- 

 pace; lit, tieart; int, intestine; ng, nervous cord. 



by the Leriueaoi the cod (Fig. 104) or the fish-louse of the 

 menhaden (Fig. 105). 



Order 3. BrancMopoda (Bivalved Crustacea).— All the 

 Crustacea hitlierto mentioned breathe through their skin, 

 having no gills; we now come to Crustacea whose body is 

 protected by a rather thick shell or carapace, and which 

 breathe by gills attached to the legs, or by broad gill-like 

 expansions of the legs. In tliis order the number of seg- 

 ments varies from about 12 to GO; and tlie shield or carapace 

 mostly covers the legs. Fig. 106 represents a section 

 through the body of Ajncsj is the carapace concealing 



