Rupert Jones^ on Bivalved Entomostraca. 43 



except that the last ends with the caudal flaps (post-abdomen 

 or tclson). 



Others also of these lower Crustacea, or Phyllopoda (whether 

 bivalved or not), have more than twenty segmented parts in 

 their body ; but of the twenty theoretical typical somites or 

 segments (twenty-one,* including the telson) characteristic of 

 a well-developed Crustacean, several of the hindmost are 

 absent in most of the Bivalved Entomostraca ; and this cur- 

 tailed form is wholly enveloj^ed in the two more or less 

 closely fitting carapace-valves of the cephalothorax. 



Thus in the Phyllopodous Liinuadia, after the front part 

 of the body, bearing the antennae, eyes, and mandibles, suc- 

 ceed twenty-two pairs of branchial limbs, more or less de- 

 veloped, followed by the post-abdomen. Locomotion is here 

 effected by the antennae and post-abdomen. In the Cladoce- 

 rous (Daphnioid) and Ostracodous (Cyproid) groups, how- 

 ever, of the Entomostraca, the antennae, eyes, mandibles, and 

 maxillae, two to six pairs of feet (Avith branchial appendages 

 attached to some of them), a short abdomen, and a strong, 

 hooked post-abdomen, are the chief features ; so in these 

 Bivalved forms, instead of the numerous branchial laminae of 

 the Phyllopods, Ave have a few pairs of locomotive organs 

 w'ith their branchial appendages. 



The disposition of the organs in various orders, families, 

 and genera, may be studied in detail in the works of Baird, 

 Dana, Zenker, Lilljeborg, Eischer, Grube, Sars, Norman, 

 Brady, and others. For the family and generic characters of 

 the Ostracoda, see G. S. Brady's memoir in the ' Intellectual 

 Observer' for September, 1867; and for the specific charac- 

 ters of many of the Cladocera, see Xorman and Brady's 

 memoir on the Bosminidte, &c., in the ' Nat. Hist. Trans. 

 Northumberland and Durham,' I8GT. 



The Bivalved Entomostraca diffbr among themselves not 

 only Avitli 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 doAvn, 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, 



* The twenty-one ilieoretical somites are thus allocated by some natu- 

 ralists : — seven to the head or cephalon, seven to the thorax or pereion, and 

 seven to the abdomen or pleon. 



