xi PHYLUM ARTHROPODA 673 



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 consists 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. 



APPENDIX TO THE ARACHNIDA. 



THE PYCSOGONIDA, LINGUATULIDA, AND TARDIGRADA. 



These three groups, though not in any way related to one another, and of 

 doubtful relationships to the Arachnida, are, as a matter of convenience, men- 

 tioned together here. 



The Pyonogonida. 



These are marine Spider-like Arthropods (Fig. 558) in which the body consists 

 of a cepthalothorax composed of an anterior proboscis (s), three head-segments, 

 and one thoracic segment, followed by three free thoracic segments and a rudi- 

 mentary abdomen (ab.). The cephalothorax bears usually four simple eyes and 

 four pairs of appendages, one or both of the first two of which may be chelate. 

 To these succeed a pair of usually ten-jointed ovigerous legs (3), and the first 

 pair of thoracic legs (4). The free thoracic segments bear lateral processes for 

 the articulation of the remaining three pairs of legs. The rudimentary abdomen 

 (ab.) is devoid of appendages. 



Diverticula from the mesenteron penetrate for a considerable distance into 

 the limbs. Malpighian vessels are absent. There is a tubular heart with two or 

 three pairs of ostia. Organs of respiration are absent. The nervous system 

 consists of brain, sub-cesophageal ganglia and three other ganglia in the cephalo- 

 thorax, and one or two small pairs in the abdomen. The testes in the male 

 are partly, and the ovaries in the female either partly or completely contained 

 in the bases of the thoracic appendages on which they open. In the male 4-7 

 cement-glands are situated in the fourth joints of certain of the appendages ; 

 their secretion cements the eggs together into masses which are carried on the 

 ovigerous legs of the male, and in one species on those of the female also. 



A metamorphosis occurs in most cases. The larva usually has three pairs of 

 appendages, so that it bears a superficial resemblance to a nauplius ; but the 



