ANNULOSA : AEACHNIDA. 



199 



parts which bear the same name in the other Arthropoda, but 

 of the antennce. The antennae, therefore, of the Spiders are 



Fig. 57. Morphology of Arachnida. 1. Organs of the mouth in the Scorpion, 

 on one side ; m. Mandibles (antennae), converted into chelce, and called 

 the cheliceraj ; 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 ' stigmata ' or apertures of the pulmonary sacs. 



4. Tegenaria domestica, the common Spider (Male), viewed from below ; 



5. Spinnerets ; m. Mandibles with their perforated hooks ; below the 

 mandibles are the maxillaa, and between the bases of these is the labium ; 

 p. The maxillary palpi with their enlarged tumid extremities. 



converted 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 enveloped in a 

 sheath, formed by the labium and maxillas, whilst the man- 

 dibles are often joined together so as to constitute a species of 

 lancet. 



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 

 and straight, no convolutions intervening between the mouth 

 and the aperture of the anus. 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 



