62 INVERTEBRATE ANIMALS. 



Sub-class ENTOMOSTRACA. 



In the third or highest Sub-class of Crustacea, 

 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. xcuttyi, an oar ; ttouc, a foot. 

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

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

 Lerncea, Achtheres, Penimdus, and various other genera 

 parasitic on fishes, are recognised by Professor Huxley 

 as peculiarly modified Copepoda. 



III. Order Cladocera. jcAaSoj, a branch ; xepag 

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



IV. Order Phyllopoda. <^6xKov, a leaf; ttouj, 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 Bero'e 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. 



