ARACHNIDA. 575 



former being tubular and the latter pouch-like in form. The air is 

 admitted to the breathing-organs by paired apertures ("stigmata"), 

 which are mostly placed on the ventral aspect of the abdomen. 

 Many of the lower Arachnida have no differentiated respiratory 

 organs ; and the Pycnogonids, if rightly placed in this class, are 

 water-breathers. 



As regards their distribution in space, the higher Arachnida are 

 essentially terrestrial in habit. Many of the lower forms are inter- 

 nal or external parasites. A few of the Mites live in the sea or 

 between tide-marks, and the singular Pantopods (Pycnogonida) are 

 wholly marine in habit : but the zoological position of these last is 

 uncertain. 



As regards their distribution in time, our knowledge of the history 

 of the Arachnida is still very incomplete. Many of the lower 

 forms are incapable of preservation, while the terrestrial habits of 

 most of the higher forms sufficiently account for their comparative 

 rarity as fossils. The oldest known representatives of the class are 

 the Scorpions, which appear, under forms not very widely different 

 from those now in existence, in the Silurian rocks. The earliest 

 types of the true Spiders (Protolycosa and Phalaranea) appear in 

 the Coal-measures, where also appear forms allied to the recent 

 Thelyphonus. The great majority of the Palaeozoic Arachnids, 

 however, differ in various points from existing forms, and have 

 been placed by Scudder in a special order (Anthracomarti), which 

 is confined to the Palaeozoic period. In the Mesozoic rocks no 

 undoubted remains of Arachnids have hitherto been detected. On 

 the other hand, in the Tertiary rocks are found representatives of 

 all the living orders of Arachnids, except the problematical group 

 of the Pycnogonida^ which is not known at all in the fossil state. 

 Most of the Tertiary Arachnids have been found in amber — the 

 fossil resin of certain Conifers — and the state of preservation of 

 these is often marvellously perfect. The more important facts 

 relating to the fossil Arachtiida may be briefly noticed under 

 the following heads : — 



i. Acarida. — This group of the Arachnida comprises the Mites 

 and Ticks, in which the cephalothorax and abdomen are fused into 

 a single mass, while the mouth-organs are generally adapted for 

 piercing and suction. Breathing organs may be wanting, but 

 when present, are in form of tracheae. All the principal existing 

 families of Mites are known to be represented in amber (Tertiary). 

 Galls formed by Mites have also been detected on the leaves of 

 fossil Willows in the Tertiary beds of Europe. 



2. Anthracomarti. — This order has been founded for the re- 

 ception of certain Palaeozoic Arachnids, in which the body is some- 

 what compressed, the cephalothorax and abdomen are distinctly 



