140 INVERTEBRATE ANIMALS. 



Crustacea. In the lower Arachnida^ such as the ticks, the 

 organs of the mouth are modified to enable them to imbibe 

 fluids. 



The mouth in the Arachnida opens into a pharynx, which 

 is of extraordinarily small diameter in the true spiders, which 

 live simply on the juices of their prey. The intestinal canal 

 is usually short and straight, and is continued without convo- 

 lutions to the aperture of the anus. Salivary glands are also 

 present, as well as ramified tubes which are believed to act as 

 kidneys. 



The circulation is maintained by means of a dorsal heart, 

 which is situated above the alimentary canal. All the Arach- 

 nida breathe air directly, and the function of respiration is 

 performed by the general surface of the body (as in the lowest 

 members of the class), or by branched air-tubes termed "tra- 

 cheae," or by distinct pulmonary chambers or sacs, or, lastly, 

 by a combination of tracheae with pulmonary sacs. The tra- 

 cheae are essentially similar in structure and function to the 

 breathing-tubes of the Myriapoda and Insecta, and consist of 

 tubes, which open on the surface of the body by distinct aper- 

 tures called " spiracles." The walls of the tube are prevented 

 from collapsing by means of a spirally-coiled thread or filament 

 of chitine, which is wound round their walls within their inner 

 lining. The pulmonary sacs which occur in the Arachnida 

 are simple chambers formed by an inversion of the skin, which 

 constitutes a number of closely-set plates or folds. The whole 

 of the interior of the pulmonary sacs is richly supplied with 

 blood, and air is admitted by means of minute external open- 

 ings. 



The nervous system is of the regular articulate type, but 

 the ganglia of the ventral chain are often massed together in 

 particular situations. In no case are compound eyes present; 

 and, when distinct organs of vision exist, these are in the form 

 of from two to eight simple eyes. 



ORDERS OF THE ARACHNIDA. 



ORDER I. PODOSOMATA. In this order are included the 

 " Sea-spiders," which are wholly marine, and were long be- 

 lieved to be referable to the Crustacea on this account. As 

 they have no respiratory organs of any kind, the question can- 

 not be definitely settled, but they have no more than four 

 pairs of legs, and would therefore seem to be properly refer- 

 able to the Arachnida. In some forms the legs attain an ex- 



