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CRUSTACEANS. 
which the abdominal appendages are modified for breathing air. Like all 
crustaceans that have adopted a terrestrial life, they seem able to live only in air 
saturated with moisture. The body is usually broadly oval, convex above, and 
flat or hollow beneath, widest in the middle, and gradually narrowing towards 
the head and tail. The head is small, but the thorax large and seven-jointed, the 
abdomen being short. Representatives of this tribe are found in all quarters 
of the globe. A familiar British species is the sea-slater ( Ligia oceanica), a 
large species living amongst the stones and rocks upon the coast above high water. 
The creature is nocturnal, and unless disturbed is not often seen during the day, 
but issues from the cracks and clefts of rocks in numbers at night. More obtrusive 
are the common wood- 
lice Porcellio scaber and 
Onisciis asellus, the for¬ 
mer distinguishable from 
© 
the latter by its duller 
colour and the granules 
upon its segment; Oniscus 
being smooth and more or 
less variegated. Both 
these are rather flat, and 
incapable of rolling up 
into a ball; but the pill 
wood - louse ( Armadil- 
lidium vidgare) has the 
dorsal surface more con¬ 
vex, and when handled rolls up into a ball. On account of their resemblance 
to pills, these creatures were used for various maladies. In addition to its rounder 
shape, the pill wood-louse may be recognised by the fact that the appendages 
of the last abdominal segment (the uropods) do not project like a couple of 
small tails from the hinder end of the body. Members of this group, differing 
but little from the species described, are widely distributed in all temperate and 
tropical parts of the world. 
The tribe Phreatoicidea can only be briefly noticed. It contains the genus 
Phreatoicns, of which two species—both inhabiting fresh water—are known, one 
from New Zealand, and the other from Australia. The body is long and laterally 
compressed, the seven thoracic appendages are well developed, and the first is sub¬ 
chelate as in many Amphipocla, and the abdomen consists of six distinct segments, 
with the gills attached to its appendages. The last tribe, Chelifera, containing the 
genus Tanais and others, approaches the next order, and is distinguished by 
having the first pair of appendages following the jaws—that is the second maxilli- 
pedes—pincer - like. It further differs in that the abdominal limbs are used 
rather for swimming than for respiration; the breathing-chamber is situated 
in the posterior portion of the thorax, and a constant circulation of water is 
kept up within it by the movement of a process projecting backwards from the 
maxillipedes. 
common and pill wood-lice (nat. size). 
