ANNULOSA I ARACHNIDA. 255 



normal pairs of antennae is never developed, and the eyes are 

 always sessile; while, in the higher Crustacea, the eyes are 

 mounted upon movable peduncles, and both pairs of antennae 

 are developed" (Huxley). 



The head in the Arachnida is always amalgamated with the 

 thorax, to form a " cephalothorax ; " the integument is usually 

 chitinous, and the locomotive limbs are mostly similar in form 

 to those of insects, and are usually terminated by two hooks. 



In many of the Arachnida the integument remains soft over 

 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 

 integument 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 

 " epimera ; " whilst the latter is made up of a median " ster- 

 num " and of two lateral " episterna." 



As regards the composition of the cephalothorax of Spiders, 

 "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-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 appendages 

 (fig. 98, 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. 98, 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. 



