DAPHN1A. 129 



is no separate head, this being united to or conso- 

 lidated with the first joint of the thor'ax or chest, the 

 head and thorax together comprising fonr joints. The 

 remaining joints enclose the belly, or abdomen, which 

 has the appearance of a tail ; but the tail is constituted 

 by the two last parallel pieces, which are furnished 

 with fine feathery filaments. 



The female is most commonly met with, and is 

 easily known by having the egg-pouch, or ovary (o) 

 external on each side, and filled with eggs or ova. 

 The little Cyclops is readily recognized by its form and 

 jerking motion through the water. 



Daph'nia pulex (PI. IX. fig. 31) is a very common 

 Entomostracan, and is very well adapted to illustrate 

 the structure, on account of its size and transparence. 

 In this animal the body is loosely connected with a 

 bivalve shell, which, on careful examination, is seen to 

 be reticulated or marked with net-like lines. The 

 superior antennae (a) are very small, placed under a 

 small beak, and have at the end a minute tuft of hairs. 

 The inferior antennae (b) resemble arms, being large 

 and branched ; and by means of them the animal rows 

 itself through the water. The structure of the eye is 

 curious, consisting of a number of round lenses aggre- 

 gated together, the fine muscular threads by which it 

 is moved being easily distinguished with a high power. 

 The legs are flattened, and furnished with elegant 

 feathery setae (seta, a bristle), serving as gills or 

 branchiae. They are constantly in motion, fanning 

 the water so as to change incessantly the portion with 

 which they are in contact. About the middle of the 

 back is placed the little transparent heart, with its 

 colourless blood, which may be distinctly seen beating, 

 or contracting and dilating, in the living animal ; and 

 between the back of the animal and the shell are seen 

 the ova, which remain there until they are hatched. 



The genera and species of the Entomostraca are 

 very numerous. Those mentioned above will serve 



