318 MANUAL OF ZOOLOGY. 



into prehensile and offensive weapons; whilst in the Scorpions, 

 as in the King-crabs, they are developed into nipping-claws, or 

 chelae. 



In the lower Arachnida, the organs of the mouth, though 

 essentially the same as in the higher forms, are often enveloped 

 in a sheath, formed by the labium and maxillae, whilst the 

 mandibles are often joined together so as to constitute a species 

 of lancet. 



The mouth conducts by an oesophagus, sometimes by the 

 intervention of a pharynx, to the stomach, which often carries 

 longer or shorter caeca appended to it. The intestinal canal 

 is short and straight, no convolutions intervening between the 

 mouth and the anus. The terminal portion of the intestine 

 is generally dilated into a cloaca, into which open, as a rule, 

 branched or tortuous tubes, supposed to have a renal function, 

 and to correspond with the " Maipighian vessels " of Insects. 

 Salivary glands are generally present, and there is usually a 

 well-developed liver. 



The circulation in the Arachnida is maintained by a dorsal 

 heart, which is situated above the alimentary canal, and is 

 wanting in the lower forms. Usually the heart is greatly 

 elongated, and resembles the "dorsal vessel" of the Insecta. 

 In the lower Arachnida, however, there is no central organ 

 of the circulation, and there are no differentiated blood-ves- 

 sels. All the Arachnida, except some of the lowest, breathe 

 the air directly, and the respiratory function is performed by 

 the general surface of the body (as in the lowest members of 

 the class), or by ramified air-tubes, termed " tracheae," or by 

 distinct pulmonary chambers or sacs ; or, lastly, by a combina- 

 tion of tracheae and pulmonary vesicles. The "tracheae" con- 

 sist of ramified or fasciculated tubes, opening upon the surface 

 of the body by distinct apertures, called " stigmata." The walls 

 of the tube are generally prevented from collapsing by means 

 of a chitinous fibre or filament, which is coiled up into a spiral, 

 and is situated beneath their epithelial lining. The pulmonary 

 sacs, or " tracheal lungs," are simply involutions of the integu- 

 ment, abundantly supplied with blood ; the vascular surface 

 thus formed being increased in area by the development of a 

 number of close-set membranous lamellae, or vascular plates, 

 which project into the interior of the cavity. Like the tracheae, 

 the pulmonary sacs communicate with the exterior by minute 

 apertures, or "stigmata" (fig. 162, 3), and they are to be re- 

 garded as being simply greatly expanded tracheae. 



The nervous system is of the normal articulate type, but is 

 often much concentrated. Typically there is a cephalic or 



