194 



Elementary Zoology. 



are two pairs of antennae. The Crustacea are divided into two 

 groups as follows : — 



1. Entomostraca, with usually a varying number of 

 segments. 



2. Malacostraca, with only nineteen segments. 



The Entomostraca are divisible into the four orders 

 which follow : — 



(i) Phyllopoda.— This order contains the comparatively 

 large Aptis, and the minute water-fleas (Fig. 85). There is often 



Fig. 85.— Water Y\esi (,Dafhnia pulex). 

 Magnified. 



Fig. 86.— a Cyclops, viewed from 

 tlie side. Magnified. 



a carapace in front. The number of segments may be large 

 {Apus) or small {Daphnid). The appendages bear gills. 



(2) Ostracoda. — These Crustacea are minute, and the body 

 is not clearly segmented. The body is enclosed in a bivalve 

 shell, which has adductor muscles like a Lamellibranch. There 

 are seven pairs of appendages, no gills. 



(3) Copepoda. — The free-living members of this order have 

 a segmented body without a carapace ; the abdomen has no 

 appendages. Cyclops {¥\g. 86), of our fresh waters, is an abundant 

 form. The parasites known as fish-lice belong to this division. 



(4) Cirrhipedia. — This order contains not only the bar- 

 nacles, but a number of parasitic creatures, which have become 

 so degenerate that the general definition used here only partly 



