656 



ZOOLOGY 



SECT- 



lens 

 / 



function of the threads formed from the secretion of the spinning 

 organ is to constitute the material for the manufacture of a cocoon 

 enclosing the eggs, and in some Araneids this is the sole purpose to 

 which they are devoted. In others there is added a nest for the 

 protection of the eggs and of the parent itself ; this in many cases 

 becomes a permanent lurking-place which the Spider inhabits at 

 all seasons, 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, and frequently also a nest in 

 which the Spider lies in wait. A subsidiary function of the threads 

 is to aid in locomotion, 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, burrowing 

 in sand ; their food con- 

 sists of various kinds 

 of marine Annelids. 



Geological History. The most ancient of the living groups of 

 the Arachnida are the Scorpions, which are represented in Silurian 

 rocks by various fossil forms not differing very widely from those 

 existing at the present day. The earliest known fossil Spiders 

 have been found in deposits of Carboniferous age ; and remains of 

 Pedipalpida occur in the same formation. In Tertiary deposits 

 there have been found representatives of all the principal groups 

 of living Arachnida. 



The earliest fossil-remains of Xiphosura that have been found 

 occur in strata of the Triassic period. Other fossil species occur 

 in later formations. These are all nearly related to the living 

 species of Limulus. The Eurypterida, as already noted, are 

 entirely Palaeozoic, ranging from the Lower Silurian rocks to the 

 Devonian. 



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FIG. 567. Section of the central eye of Euscorpius. 

 Letters as in preceding figure, -pigm. cells containing 

 pigment ; vtr. vitreous body (a specialised part of the 

 ectoderm). (After Lankester and Bourne.) 



