ANNULOSA : ARACHNIDA. 257 



the juices of their prey. The intestinal canal is usually short 

 and straight, no convolutions intervening between the mouth 

 and the aperture of the anus. Often, however, lateral caeca 

 are appended to the alimentary tube. Salivary glands are also 

 present, as well as ramified tubes, supposed to perform the 

 functions of a kidney, and to correspond to the " Malpighian 

 vessels " of Insects. 



The circulation in the Arachnida is maintained by a dorsal 

 heart, which is situated above the alimentary canal. 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 dif- 

 ferentiated blood-vessels. All the Arachnida, except, perhaps, 

 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 combination of tracheae and pulmonary vesicles. 

 The " tracheae " consist of ramified or fasciculated tubes, 

 opening upon the surface of the body by distinct apertures, 

 called " stigmata." The walls of the tube are generally pre- 

 vented 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 are 

 simple involutions of the integument, abundantly supplied 

 with blood ; the vascular surface thus formed being increased 

 in area by the development of a number of close set mem- 

 branous 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 " stig- 

 mata "(fig. 98, 3). 



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

 often much concentrated. In the Spiders there is a cephalic 

 or " cerebral" ganglion, a large thoracic ganglion, and in some 

 instances a small abdominal ganglion. In some of the lower 

 forms the articulate type of nervous system is lost, and there is 

 merely a ganglionic mass which is traversed by the gullet. In 

 none of the Arachnida are compound eyes present, and in none 

 are the eyes supported upon foot-stalks. The organs of vision, 

 when present, are in the form of from two to eight or more 

 simple eyes, or " ocelli." 



In all the Arachnida, with the exception of the Tardigrada, 

 the sexes are distinct. The great majority of the Arachnida 

 are oviparous, and in most cases the larvae are like the adult in 

 all except in size. In some cases, however (Acarina), the 



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