340 THE CANADIAN NATURALIST. [Sept. 



see G. S. Brady's memoir in the 'Intellectual Observer' for 

 September, 1867 ; and for the specific characters of many of the 

 Cladocera, see Norman and Brady's memoir on the Bosminidoe, 

 &c., in the ' Nat. Hist. Trans. Northumberlmd and Durham,' 1867. 



The Bivalved Entomostraca diifer among themselves not only 

 with respect to the arrangement and characters of the organs of 

 sense, mastication, locomotion, and aeration, but also very markedly 

 in the shape and structure of their carapace-valves. 



In Apus, one of the Phyllopods, the carapace (or shell covering 

 the cephalothorax) is nearly flat and shield-like, but ridged along 

 the middle. In Nebalia, another Phyllopod, the carapace is 

 folded down, as it were, on either side of the animal ; the abdomen 

 extends beyond it behind, the legs below, and the antennae in 

 front, with a small, arched, movable projection above the eyes. 

 In the Cladocera (^Daphnia, &c.) the carapace is still more flatly 

 folded down, with a bend along the dorsal line ; and the whole of 

 the body is included within it, except that the antennae (as swim- 

 ming limbs) protrude at the Jjead from lateral notches, which 

 give to the front of the carapace a hood-like or quaintly beaked 



In other Bivalved Entomostraca the two sides of the folded 

 carapace are quite distinct, forming separate valves, but united 

 in life along their dorsal margins by either a simple membranous 

 attachment (as in Estheria, &c.), or by a more complex system of 

 ridge and furrow, or teeth and sockets (as in the Cyproidea). 



In outline the carapaces of Cladocera range from orbicular to 

 oblong, with varying contours. They are horny or chitinous, 

 thin, usually transparent, and ornamented often with some reticu- 

 late pattern, having reference to the hexagonal cell-system of the 

 typical crustacean test, or the network resolves itself into the 

 delicate bands and furrows by the greater development of one 

 set of mesh-lines than another. This carapace is periodically 

 moulted and renewed ; but occasionally it is retained, and one 

 layer succeeds on the inside and at the outer edge of another 

 until the valve is marked with several concentric boundary-lines 

 of the periodic stages of growth. Mr. Norman points out that 

 this feature, normal in Menosphllus tenuiwstris, is occasional in 

 Lynceus elongatus ; see ' Nat. His. Trans. Northumberland and 

 Durham,' 1867, p. 53. It is also normal in the Limnadiadce^ 

 which retain their valves, whilst they cast only a chitinous 

 skeleton or framework of the body. 



