262 MANUAL OF ZOOLOGY SECT, x 



Trap-door Spider, the nest has a closely fitting hinged lid. 

 In very many Spiders the secretion is used mainly to form 

 the web by means of which the prey is snared, with the 

 addition frequently of a nest in which the Spider lies in 

 wait. A subsidiary function* of the threads is to aid in loco- 

 motion, the Spider being enabled by means of them to let 

 itself down safely from considerable heights, and even to 

 float in the air. 



Some of the Mites, as already mentioned, are parasitic ; 

 others feed on various kinds of fresh or decaying animal or 

 vegetable substances. Most free Acarida are terrestrial ; 

 some are aquatic. 



The Xiphosura are marine, living at a depth of a few 

 fathoms in warm seas, and burrowing in sand ; their food 

 consists of various kinds of marine Annelids. 



