198 MANUAL OF ZOOLOGY. 



the entire body : in others, as in the majority of Spiders, the 

 abdomen remains soft and flexible, whilst the cephalothorax is 

 more or less hard and chitinous ; in the Scorpions, again, the in- 

 tegument over the whole body forms a strong chitinous shell. 



The typical somite of the Arachnida is constituted upon 

 exactly the same plan as that of the Crustacea, consisting 

 essentially of a dorsal and ventral arc ; the former composed of 

 a central piece, or ' tergum,' and of two lateral pieces, or ' epi- 

 mera ; ' whilst the latter is made up of a median ' sternum ' 

 and of two lateral ' episterna.' 



As regards the composition of the cephalothorax of Spiders, 

 ' the tergal elements of the coalesced segments are wanting, 

 and fche back of the thorax is protected by the elongation, 

 convergence, 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-develop- 

 ment 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 appen- 

 dages (fig. 57, 4). 1. 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 ' lab rum,' is 

 also present. 



In the Spiders (fig. 57) 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. 57, 1) the mandibles are short, and 

 terminate in strong pincers, or ' chelicera3.' The maxillary 

 palpi are, also, greatly developed, and constitute powerful 

 grasping claws, or ' chelae.' In the genus Galeodes, the 

 mandibles, like those of the Scorpion, constitute ' chelicerae,' 

 though comparatively much larger and longer ; but the max- 

 illary palps are not developed into ' chelae.' 



With regard to antennas, these organs, as sucli, do not exist 

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

 mandibles of the Arachnida are truly homologues, not of the 



