206 ZOOLOGY. 



CLASS IV. : CRUSTACEA (CRUSTACEANS). 



The Crustacea breathe by gills, and are therefore 

 suited to an aquatic life. A few species, however, 

 live in damp earth, or in places where the air is 

 damp (wood lice). Crustacea have two pairs of 

 antennae, and a large number of appendages arranged 

 in a characteristic way, but differing very much in 

 shape in the different groups ; skin usually hard and 

 thick. Lobsters, crayfish, crabs, wood-lice, and the 

 small sand-hoppers, water-fleas, etc., belong to the 

 Crustacea. No Crustacean is harmful agriculturally. 



Third Sub-Kingdom : VERMES (Worms). 



Worms ^ are bilaterally symmetrical animals, in 

 which the body is enclosed in a " dermo- muscular 

 tube." Under the delicate epidermis there is found 

 a layer, which does not, as in the higher animals, 

 consist exclusively of dermis, but is partly composed 

 of muscle-fibres, which form a distinct coat internally. 

 Worms are able to move by contracting the various 

 components of the dermo-muscular tube thus formed. 

 In some worms limbs assist in the movements, but in 

 others this is not so ; in any case, however, the limbs 

 play a relatively subordinate part. These limbs, 

 which are only present in the bristle--worms, are small, 

 always unjointed, foot-stumps, which bear bristles. 

 Leeches and some other worms possess suckers by 

 which they can attach themselves, and move by 

 alternately contracting and extending their bodies. 



There are segmented and unsegmented worms 

 (thread-worms, liver-flukes) ; the degree of segmenta- 

 tion is also very various. In many tapeworms each 



' In ordinary language one understands by " worms " elongated, 

 cylindrical animals; and several insect-larvse (e.g. "wire worms" 

 p. 103) have the name applied to them. But to zoologists all " worm- 

 shaped animals are not worms, nor have all worms an elongated 

 cylindrical shape (liver-fluke). " 



