Sub-Class 1. Entomostraca. Order 1. Phyllopoda. 



195 



Limnadia, etc., afford a transition to tlie next order ; they are distinguislied 

 by having the carapace divided into two movable halves, provided externally with 

 a very hard cuticle, and surrounding the whole body; the valves shut together as 

 do those of Lamellibranohs, for which they might be mistaken. Further, the 

 lateral eyes are very near one another or even united ; the second antennse are 

 very strong and provided with two jointed filaments, the exopod and the distal 

 part of the endopod respectively, whilst the first antennae attain only a small 

 size. 



Order 2. Cladocera. 



The Daphnids must be regarded as peculiarly developed Phyllopoda 

 with a small number of limbs^ and a large compressed 

 bivalve carapace enclosing the body with the append- 

 ages ; upon the head there is a large compound eye, which is 

 mounted upon a short stalk and is movable ; it arises from the 

 fusion of the lateral eyes, and is enclosed in a special socket, formed 

 by the upgrowth of a fold of skin ; there is usually also a small 

 unpaired nauplius-eye. The first antennsB are generally short, and 

 provided with olfactory hairs. The second antennae are 

 powerful biramous natatory organs. Besides the mandibles 



Fig. 156. A Daphnid, Stda CTj/s'aHina, with eight winter eggs in the brood-pouoh. 

 a, first, a^ second, antenna, an anus, d gut, h heart, m mouth, o eggs, oe eye, ov ovary, 

 r caudal fork, S carapace. — After "Weismann. 



there is a feebly developed pair of maxillge. The short thorax is 

 provided with laminate nectopods like the limbs of the Phyllopoda, 

 though there are only four to six pairs. The abdomen is curved 

 dovmwards, and has two pointed, unsegmented caudal appendages at 

 the tip. There is a powerful pulsatile heart anteriorly and dorsally, 



2 



