no 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 is unsegmented and never bears any limb pro- 

 cesses. The tracheae, instead of being bundles of branch- 

 ing tubes, are condensed and flattened, and included in 

 definite spaces, in which the compressed tubes look like 

 FIG. 61. the leaves of a book, the whole laminated 



organ, on account of its being circum- 

 scribed and lung-like, is called a trachea I 

 lung, and the spiders are often called pul- 

 monary arachnoids on account of their 

 possessing two or four of 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 

 antennary jaws have at their inner 

 side the duct of a poison-gland whose 

 secretion they instil into the insects which constitute 

 their prey. The stomach is like a hollow ring from 



