

XIl] OSTRACODA 209 



Mussel, of two valves. It opens by an elastic ligament which tends 

 to pull the valves apart, and it closes by the contraction of a muscle 

 which runs across the body from one valve to another. The whole 

 body is included in the carapace, antennae and all. 



Ostracoda have fewer appendages than any other group of 

 Crustacea; besides the antennules, antennae, mandibles and two 

 pairs of maxillae, they possess only two pairs of limbs, and these 

 are stout and cylindrical in marked contrast to the appendages 

 of the Phyllopoda. The hinder part of the body is rudimentary. 



Two pairs of excretory organs have been described in some 

 species of Ostracoda, the shell-glands common to all Entomostraca 

 opening at the base of the second 

 maxillae and a pair of an ten nary 

 glands opening at the base of the 

 second antennae. The last named 

 are seldom found except in the 

 Malacostraca. 2 ( 



Both pairs of antennae are 

 used in swimming, another 

 most important distinction from FIG. 80. Lateral view of Cypris can- 

 Daphnia and its allies. dida - After Zenker. 



The males differ from the l - Antennules. 2. Antennae. 



females, which either (Cypris) 3 ' Mandibles - 4 - 1st maxillae. 



1,1. 5. 2nd maxillae. 6. 1st pair 

 lay their eggs on water-plants , , , . e , 



fn -7- N of legs. 7. 2nd pair of legs, 



or (^Lypnaina) carry them about 8 T^ E 



within their shells. The majority 



of species are found in the sea but others occur in fresh water. 

 They are flesh eaters, and as they exist in great numbers they 

 fulfil the important duty of scavenging on a small scale, and thus 

 they prevent the accumulation of dead organic matter in the water. 

 They are divided into two main divisions: (A) Podocopa with 

 unbranched second antennae, inhabitants of shallow shore waters 

 which do not swim strongly but creep a good deal. (B) Myodocopa 

 with powerful forked second antennae and a notch in the edge of 

 each valve of the carapace to allow the antennae to be vigorously 

 moved ; they are inhabitants of deeper waters. Fossil Ostracoda are 

 found in the oldest fossiliferous rocks. 



Order III. Copepoda. 



This order Gr. KOJTT^, oar ; TTOV?, TroSo'g/foot) is also a large one and 

 its free-swimming members exhibit a very characteristic structure and 



S. & M. 14 



