HO Invertebrata. 



headed-worms. They are however easily distinguished 

 by their embryos bearing true jointed limbs, although 

 these are lost in the adults. 



One form has been found in the contents of the 

 small fat glands on the human face, and another is 

 the cause of the disgusting skin disease known as 

 ' itch/ Other larger forms are the ' ticks ' found so 

 commonly on sheep, dogs, bats, camels, &c. Of 

 non-parasitic forms, the little ' red-spider' so often 

 seen on the sea-shore under stones between tide- 

 marks, and the ' glass- ' and ' garden-mites ' found in 

 damp moss and among vegetables are examples. 



Spiders. In spiders the cephalo-thorax is joined to 

 the sac-like abdomen by a narrow stalk, and the latter 

 portion never bears any limb-processes. The tracheae, 

 instead of being bundles of branching tubes, are con- 

 densed and flattened, and included in definite spaces, 

 in which the compressed tubes look like the leaves of 

 FIG. 61. a book, the whole laminated organ on 

 account of its being circumscribed and 

 lung-like, is called a tracheal-lung, and 

 the spiders are often called pulmonary 

 arachnoids on account of their possess- 

 ing these organs. 



Spiders have little clusters of simple 

 eyes on their foreheads, bright small 

 specks usually eight in number and 

 generally arranged in two rows. The 

 mandibles have at their inner side 



Scorpion. ^ ^ uct Q f a p O i son _gl an d whose 



secretion they instil into the insects which constitute 

 their prey. The stomach is like a hollow ring from 



