xii CRUSTACEA 171 



those of the thorax, and the abdomen is much reduced in 

 size. 



Wood-lice, or Land-slaters, are adapted to some extent 

 to life on land, for their abdominal legs have been shown 

 to contain little air-tubes somewhat like the " tracheae " 

 of insects. Although they can breathe air they are still 

 dependent on moisture, without which they speedily dry and 

 die ; they are therefore usually found living beneath a stone 

 or a log of wood, or in some other sheltered and damp spot. 



The Common Garden Wood-louse, or " Pill " Wood-louse 

 (Armadillidium vulgare), is easily recognised by its habit of 

 rolling up into a ball when touched. 



The White Slater (Platyarthrus Ho/mannseggii) (Fig. 107) is 

 a much smaller, but fairly common form, 

 not more than ^ of an inch long ; it is 

 notable because it always lives in the 

 nests of ants, apparently welcomed and 

 cared for by them (see p. 416). These 

 white " Slaters " are quite blind, and they 

 have very short antennae. 



LOWER CRUSTACEA (ENTOMOSTRACA) FIG 107 _ The white 



A n i T f* -,, Slater (Platyarthrus 



AM the Lower Crustacea are small 



primitive forms in which the segmenta- (Much enlarged.) 

 tion of the body and the form of the 

 appendages are very variable. They often have a single 

 median simple eye as well as two lateral compound eyes. 



Order 1 : BRANCHIOPODA 



The Water These little Crustaceans are very minute, and 

 Flea, extremely transparent, but in spite of this they can 



Daphnia. j^ reac [ily seen i n the water of a clear pond, as 

 they swim up and down with a curious jerky movement in the 

 early morning and the evening ; during the hotter hours of 

 the day they rest on the mud bottom. The name of the order 

 refers to the leaf-like swimming appendages (Fig. 108). 



The commonest species is Daphnia pulex. The cara- 

 pace is compressed laterally, so that it forms a covering 

 like a bivalve shell ; it has a spine projecting backwards from 



