7. PHYLUM ARTHROPODA 263 



others there is added a nest for the protection of the eggs 

 and of the parent itself; this in many cases becomes a per- 

 manent lurking place which the spider inhabits at all sea- 

 sons, and from which it darts out to capture its prey; in the 

 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 from low-water mark to 

 the depth of a few fathoms in warm seas, burrowing in sand ; 

 their food consists of various kinds of marine annelids. 

 Limulus polyphemus inhabits our coast from Florida to, and 

 just east of the mouth of, the Kennebec River, Maine. 



