ANNULOSA: ARACHNIDA. 



223 



The mouth opens into a pharynx, which is of remarkably 

 small calibre in the true spiders, all of which live simply on 

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



\ 4 



Fig. 74. Morphology of Arachnida. i. Organs of the mouth in the Scorpion, on 

 one side ; m Mandibles (antennae) converted into chelae, and called the chelicerae ; 

 p Maxillary palpi greatly developed, and forming strong chelae. 2. Telson of the 

 Scorpion. 3. One of the abdominal segments of the Scorpion, showing the "stig- 

 mata," or apertures of the pulmonary sacs. 4. Tegenaria domestica, the common 

 Spider (male), viewed from below ; ^ Spinnerets ; m Mandibles with their perforated 

 hooks below the mandibles are the maxillae, and between the bases of these is the 

 labium ; / The maxillary palpi with their enlarged tumid extremities. 



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 differ- 

 entiated blood-vessels. All the Arachnida 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 dis- 

 tinct 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 



