572 



ZOOLOGY 



SECT. 



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

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

 are small, the antennae (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 



ant. 2 



2. Polyphemus 



1 . D a h n i a 



a. 449. Throe Cladocera. ant. 1, antennule ; ant. ,?, antenna ; br. brain ; l>i-.p. brood-pouch ; 

 E. eye ; d.f/f. digestive gland ; ./'. swimming-feet ; 7^. heart ; md. mandible ; sih.riL shell- 

 gland. (1 after Glaus, L> and 3 after Gerstaecker.) 



shape of the head, the immense antennre, and the great hump-like 

 brood-pouch. 



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

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

 genera is Gypris, which occurs in immense numbers in stagnant 

 pools. Cythere is a common marine form. 



The body (Fig. 450) 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), which may be variously ornamented or sculptured. The 

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



