144 THE FOOD OF ANIMALS 



LOWER CRUSTACEA (Entomostraca) 



The innumerable marine and freshwater Crustacea which 

 come under this heading, and which are commonly of small or 

 very small size, mostly feed upon minute organisms belonging 

 indifferently to the plant or animal kingdoms, and therefore do 

 not fall under this section. Two of the included orders, however, 

 embrace parasitic species, i.e. the Copepoda, to which belong 

 " fish-lice ", and the Cirripedia or Barnacle order, in which are 

 placed a number of strangely-shaped forms parasitic upon other 

 Crustacea, or it may be upon animals belonging to other groups. 

 These will receive mention when the question of Parasitism is 

 discussed. 



KING-CRABS (Xiphosura) 



Having dealt with a few of the carnivorous forms belonging 

 to the Crustacea, we come to the second class of gill-bearing 

 Arthropods, i.e. the Xiphosura, including only the recent King- 

 Crabs (see vol. i, p. 423), regarding the classificatory position, 

 of which there has been much discussion. They inhabit shallow 

 water along the eastern coast of the United States and the 

 south-east of Asia. The animal is of considerable size, and is 

 covered on its upper side by firm protective armour, there being 

 a strong horse-shoe-shaped shield (whence the popular name of 

 "horse -shoe crab") extending over head and thorax, and a 

 large plate on the abdomen, to the hind end of which a long 

 movable spine is attached. Upon the under side of the body 

 the mouth is seen as a longitudinal slit surrounded by the rough 

 and spiny bases of the six pairs of walking legs, all of which, 

 except the last, are, in the female, provided with pincers, while 

 in the male the first pair present the common arrangement of 

 a seizing organ in which the end-joint can be turned back upon 

 the rest of the limb. The rough bases of the limbs act as jaws, 

 and this is probably to be looked upon as the simplest case of 

 the modification of a limb for this purpose, since the original 

 use as a leg is still retained, though in this particular instance 

 chewing duties have also been added. The foot-jaws or maxilli- 

 pedes, of which examples have been described among the 

 Crustacea (see pp. 141 and 142), represent a still further stage in 

 the development of jaws, where the locomotor function has been 



