2 7 6 



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 

 Oniscus 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 vulgar e) 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 

 Phreatoicus, 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 Amphipoda, 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). 



