558 



ZOOLOGY 



SECT. 



paired eyes (E) have fused into a single organ, which exhibits 

 a constant trembling movement. The antennules (ant. 1) 

 are small, the antenna? (ant. 2) very large, biramous, and con- 

 stitute the chief organs of locomotion. The mandibles are large, 

 the second maxillae absent in the adult, and there are usually 

 five pairs of leaf-like swimming-feet (/) on the thorax. The 

 abdomen is devoid of appendages. Many of the Cladocera have 

 an extraordinarily grotesque form (2, 3), owing to the peculiar 

 shape of the head, the immense antenna?, and the great hump-like 

 brood-pouch. 



CLTlt.Z 



1 . D a b h n i a 



FIG. 456. Three Cladocera. ant. 1, antennule ; ant. 2, antenna ; br. brain ; br.p. brood- 

 pouch ; E. eye ; d.gl. digestive gland ; /. swimming feet ; ht. heart ; md. mandible ; 

 sh.gl. shell-gland. (1 after Claus, 2 and 3 after Gerstaecker.) 



The Qstracoda are usually not more than 1-2 mm. in length, 

 and are found both in fresh- and sea- water. One of the commonest 

 genera isjtypzis, which occurs in immense numbers in stagnant 

 pools. Cythere is a common marine form. 



The body (Fig. 457) is unsegmented, and is completely enclosed 

 in a carapace (A), the right and left halves of which are articu- 

 lated together along the dorsal edge so as to form a bivalved shell 

 (C) t which may be variously ornamented or sculptured. The 

 valves are opened by the elasticity of a ligament, which passes 

 from one to another at the hinge, and are closed by a large adductor 

 muscle (m.), which extends transversely from valve to valve, its 



