1869.] GEOLOGY AND MINERALOGY. 339 



In the Entomostraca, only six thus coalesce for the senses and 

 mouth in the Cyclops group, only ^ue in the Daphnia and Caligus, 

 and on\j four in Limulus. 



The essential points in the framework of the body of an Ento- 

 mostracan of low organization, and in the arrangement of the 

 organs, are well seen in the Brine-shrimp {Artemia.) Here the 

 body has numerous articulations or segmented portions. The 

 head-part takes up four or five coalesced somites, bearing the 

 antennae, eyes, and masticatory organs ; eleven pairs of natatory 

 and branchial limbs follow on eleven segments; the next two 

 joints or rings have their own modified appendages; seven seg- 

 ments succeed, without appendages, except that the last ends 

 with the caudal-flaps (post-abdomen or telson.) 



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-deve- 

 loped Crustacean, several of the bindmost are absent in most of 

 the Bivalved Entomostraca ; and this curtailed form is wholly 

 enveloped in the two more or less closely -fitting carapace- valves 

 of the cephalothorax. 



Thus in the Phyllopodous Linmadia, after the front part of 

 the body, bearing the antennae, eyes, and mandibles, succeed 

 twenty-two pairs of branchial limbs, more or less developed, 

 followed by the post-abdomen. Locomotion is here effected by 

 the antennae and post-abdomen. In the Cladocerous (Daphnioid) 

 and Ostracodous (Cyproid) groups, however, of the Entomostraca, 

 the antennae, eyes, mandibles, and maxillae, two to six pairs of 

 feet (with 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, we have a few pairs of 

 locomotive organs with their branchial appendages. 



The disposition of the organs in various orders, fiimilies, and 

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

 Zenker, Lilljeborg, Fischer, Grube, Sars, Norman, Brady, and 

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



* The twenty-one theoretical somites are thus allocated by some 

 naturalists : — seven to the head or cephalon, seven to the thorax or 

 pereion, and seven to the abdomen or pleon. 



