436 AEAC1IXIDA. 



very different grades of development. Visceral nerves have been 

 shown to exist in the Spiders and Scorpions. The sense organs are, 

 as a rule, not so highly developed as in the Crustacea, and, putting on 

 one side the tactile function of the extremities, are confined to eyes. 

 The eyes are simple and immovable, and never possess a facetted 

 cornea ; they are from two to twelve in number, and are sym- 

 metrically arranged on the anterior surface of the cephalo-thoracic 

 shield. Auditory organs have not yet been discovered, but there 

 are tactile and olfactory organs. 



The alimentary canal runs straight from the mouth to the hind 

 end of the body, and is divided into a narrow oesophagus and a wide 

 intestine, which is, as a rule, provided with lateral caeca. The intes- 

 tine is, in the Spiders and Scorpions, divided into an anterior dilated 

 portion the so-called stomach and the intestine proper. The 

 glandular appendages of the digestive canal are salivary glands ; in 

 Spiders and Scorpions, a liver, composed of a number of branched 

 canals ; and, with a few exceptions, Malpighian tubes, which function 

 as urinary organs and open into the hind end of the intestine. 



The organs of circulation and respiration also show very different 

 degrees of development and are only absent in the lowest Mites. 

 The heart lies in the abdomen, and is a long, many-chambered dorsal 

 vessel with lateral slits through which the blood enters. It is fre- 

 quently continued into an anterior and posterior aorta, and in 

 Scorpions gives off in addition lateral branching arteries. The organs 

 of respiration are internal air chambers, which have the form either 

 of ramified tubes (trachea}}, or of hollow lamellae (fan-tracheae, lungs) 

 placed upon one another in great number like the leaves of a book 

 and connected together by trabecula? so as to have the form of a sac. 

 The air chambers are always kept open by a firm internal chitinous 

 membrane, so that the air can enter by the paired openings (stig- 

 mata] of the tracheae or lungs at the beginning of the abdomen, 

 and be distributed to the finest ramifications. The chitinous lining 

 may become thickened so as to give rise to a spiral fibre. 



Generative organs. With the exception of the hermaphrodite 

 Tardigrada, all the Arachnida are of separate sexes. The males are 

 frequently distinguished by external characteristics, as for example 

 by their smaller size, by the possession of organs of attachment 

 (Mites), or by the modification of certain appendages. Their genera- 

 tive organs consist of paired testicular tubes, and the vasa deferentia 

 often receive the contents of accessory glands before opening to the 

 exterior by a single or double aperture at the base (anterior end) of 



