222 MANUAL OF ZOOLOGY. 



" the tergal elements of the coalesced segments are wanting, 

 and the back of the thorax is protected by the elongation, con- 

 vergence, and central confluence of the epimeral pieces ; the 

 sternal elements have coalesced into the broad plate in the 

 centre of the origins of the ambulatory legs, from which it is 

 separated by the episternal elements The non-devel- 

 opment of the tergal elements explains the absence of wings." 

 (Owen.) 



The mouth is situated, in all the Arachnida, in the anterior 

 segment of the body, and is surrounded by suctorial or masti- 

 catory appendages. In the higher Arachnida the mouth is 

 provided from before backwards with the following appendages 

 (fig. 74, 4). i. A pair of "mandibles," used for prehension. 

 2. A pair of " maxillae," each of which is provided with a long 

 jointed appendage, the " maxillary palp." 3. A lower lip, or 

 " labium." In the Scorpion an upper lip, or " labrum," is 

 also present. 



In the Spiders (fig. 74, 4) each mandible terminates in a sharp 

 movable hook, which possesses an aperture at its extremity 

 communicating by a canal with a gland, which is placed in the 

 preceding joint of the mandible, and secretes a poisonous 

 fluid. The maxillary palps in the Spiders are long, jointed 

 appendages, terminated in the females by pointed claws, but 

 frequently swollen, and carrying a special sexual apparatus in 

 the males. 



In the Scorpions (fig. 74, i) the mandibles are short, and 

 terminate in strong pincers, or " chelicerse." The maxillary 

 palpi are also greatly developed, and constitute powerful 

 grasping claws, or " chelae." In the genus Galeodes, the man- 

 dibles, like those of the Scorpion, constitute " chelicerae," 

 though comparatively much larger and longer ; but the maxil- 

 lary palps are not developed into " chelae." 



With regard to antennae, these organs, as such, do not exist 

 in the Arachnida. It is generally believed, however, that the 

 mandibles of the Arachnida are truly homologues, not of the 

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

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

 converted into prehensile and offensive weapons ; whilst in the 

 Scorpions, as in the King Crabs, they are developed into nip- 

 ping 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 maxillae, whilst the man- 

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

 lancet. 



