190 ARACHINIDES. 



Fossil Crustacea have been found in various parts of the continent of Europe and 

 in Asia. They occur in calcareous rocks of the older formations, the chalk-formation, 

 and the later deposits. The greater part of these remains have lost their feet and an- 

 tennae, are completely changed into calcareous matter, or present the appearance of 

 casts, with a tint of brown colour. Desmarest has described thirty-six species, of 

 which twenty-one belong to the Brachyura, five to the Macroura, and ten to the 

 Branchiopoda. 



CLASS VII. ARACHNIDES. 



Oviparous animals with articulated members, and not under- 

 going a metamorphosis ; respiration tracheal or branchial, 

 the openings for the admission of the air stigmatiform ; no 

 antennae. 



THE animals of this Class were arranged by Linnaeus in the 

 last order of his class INSECTA, but were formed into a separate 

 class by Lamarck in 1 800, under the term Arachnides, from 

 aga-xyris, a spider, as denoting animals which cannot properly be 

 included either among the Crustacea or Insects. They differ 

 from the Crustacea, in having their respiratory organs always 

 in the interior of the body ; and from the -Insects in not under- 

 going a metamorphosis. 



The head in the Arachnides is not distinct from the trunk ; 

 the eyes are simple, and vary in number from two to eight. 

 Some have two jointed mandibles or forceps at the exterior ex- 

 tremity of the trunk, such as the scorpions; in others these 

 parts take the form of a sucker. The Arachnides are also desti- 

 tute of a labium or under lip, as in the insects, the part designed 

 under this name being a dilatation of the space between the 

 fore-feet, which sometimes forms part of the sucker. The mouth 

 is generally accompanied by two palpi. The number of feet 

 in the animals of this class is generally eight ; although some 

 have six, and the females of others have two additional ones 

 for the purpose of carrying their ova These feet are arrang- 

 ed round the sides of the breast, and are composed of seven 

 joints ; the first two forming the haunch, the third correspon- 

 ding to the thigh ; the two next to the leg, and the last two to 

 the tarsi. The feet are terminated by two hooks, generally den- 

 tated or pectinated below ; and a smaller simple one in the middle. 



The trunk of the body, except in one family, is soft, and with- 



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