64 INVEKTEBEATE ANIMALS. 



Sub-class ENTOMOSTRACA. 

 In the third or highest Sub-class of Ceustacea, 

 viz., Malacostraca, the post-cephalic segments are four- 

 teen in number: in the Entomostraca the number is 

 greater or less than fourteen. 



Group 161.— ENTOMOSTRACA, in part. 



I. Order Ostracoda. oa-Tpaxov, a shell. Animal- 

 cules abundant in fresh-water pools. The body is 

 enclosed in a bivalve shell. Cypris, &c. 



II. Order Copepoda. koottyj, an oar ; ttouj, a foot. 

 Water-fleas, Cyclops, &c. ; head and thorax protected 

 by a carapace. The Fish-lice, Ichthyophthira, including 

 Lerntea, Achtheres, Peniculus, and various other genera 

 parasitic on fishes, are recognised by Professor Huxley 

 as peculiarly modified Copepoda. 



III. — Order Cladocera. xXadog, a branch ; xipac, 

 a horn. Antennae used in swimming. Daphnia, &c. 



IV. — Order Phyllopoda. <puk\ov, a leaf; ttovc, a foot. 



Fairy-shrimps, &c. Apus, interesting from its affi- 

 nity to the extinct Trilobites, is said to have sixty pairs 

 of feet. Estimated number of species : recent, 190 (M. 

 Edwards) ; fossil, 70. From the Silurian upwards. 



Some of the smaller species, both fresh-water and 

 marine, swarm in numbers beyond computation. Active 

 and voracious, they in their turn become the prey of 

 enemies of many kinds ; the redness and flavour of 

 trout are supposed to be improved by a diet of Ento- 

 mostraca : the crystalline texture of Beroe discovers 

 them undergoing the process of being digested ; and 

 water-drinking animals of all orders, in the absence of 

 extreme precautions, derive some part of their nourish- 

 ment from minute Crustaceans. 



