88 
ZOOLOGY. 
abdominal segments arising after successive moults^ until ♦ 
the adult form is attained. 
Many Entomostraca 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 
en 
Fig 106 — Section through the thorax of Apus: en, 1-6, the six endites; ex, ex 
opodal or respiratory portion of the hmb forming the flabellum, /6/ c, cara- 
pace; ht^ heart; int, intestine; ng^ nervous cord. 
by the LerncBa of the cod (Fig. 104) or the fish-louse of the 
menhaden (Fig. 105). 
Order 3. BrancMopoda (Bivalved Crustacea). — All the 
Crustacea hitherto 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 this order the number of seg- 
ments varies from about 12 to 60; and the shield or carapace 
mostly covers the legs. Fig. 106 represents a section 
through the body of Apus; G is the carapace concealing 
