98 NATURAL HISTORY OF ARTHROPODS. 



Sub-Class II. — Aeachnida. 



In this division are included the mites, scorpions, harvest-men, and spiders. These 

 animals usually have the body divided into two regions, an anterior, cephalothorax, and 

 a posterior, abdomen. In the mites, however, these distinctions become obliterated, and 

 the boundaries between the regions are very indistinct. The cephalothorax bears four 

 pairs of legs adapted for locomotion, and in front of these are two pairs of mouth- 

 parts, the anterior pair being called chelicerse, or mandibles, the other the palpi, or 

 maxillae. These pairs are both post-oral, and hence we see that the antennse are lacking 

 in this group. The walking-legs are composed of a series-of joints, usually seven in 

 number; but in some, as in the harvest-men, the last, or tarsal joint, is broken up into a 

 large series of articles, while in some mites the distinctions between the joints of the 

 limbs are greatly obscured, if not wholly obliterated. 



The organs of vision, when present, are always simple eyes, placed upon the dorsal 

 surface or the sides of the cephalothorax, and varying in number from two to twelve ; 

 their number and arrangement affording characters which are largely used in separating 

 the different forms. Compound eyes, like those found among hexapods and many 

 Crustacea, are entirely lacking. 



The abdomen is without appendages except in the spiders, where the spinnerets, 

 which are frequently jointed, are homologous with the other limbs. 



The alimentary canal is nearly straight. Rarely it is a simple tube, but in most 

 forms there are a number of pockets arising from the stomach, thus increasing the 

 digestive surface. In some of the mites these pockets, or coeca, are so numerous and 

 so greatly developed as to remind one of the extensively ramified digestive tract of a 

 planarian worm. In the true spiders- these appendages of the stomach are seen in their 

 simplest form, and will be referred to again in treating of the common garden-spider. 

 In Galeodes the pockets are very long, extending for some distance into the legs, 

 reminding one of a similar extension of the alimentary tract which is found in the 

 Pycnogonids. A sucking stomach is also frequently found. Salivary glands are almost 

 invariably present, while in the higher forms a well-developed liver pours its secretions 

 into the intestine. 



The circulatory organs in the higher groups consist of a chambered heart and sev- 

 eral arteries, distributed to the various portions of the body. In the scorpions we find 

 in addition a venous system, while in the lowest mites not only blood-vessels but even 

 a heart is wanting ; the blood, propelled by the movements of the body, fio'wing between 

 the various muscles and viscera in the same way that it does in many of the lower 

 Crustacea. 



In many of the lower mites, most of which are extensively degraded by their para- 

 sitic habits, no traces of respiratory organs have yet been found, respiration being 

 effected by the whole surface of the body. In all other forms organs for the aeration 

 of the blood, in the shape of trachese or modified tracheae, are always present. These 

 tracheae may be of the normal type or they may be modified so as to form the lungs 

 which have akeady been mentioned, or both lungs and tracheai may be present 

 together. The number of pulmonary sacs varies from two in most spiders and the 

 whip-scorpions, and four in the Mygalida, to eight in the scorpions. 



