ANNULOSA: ARACHNIDA. 315 



heart, when present, is always situated on the opposite side of 

 the alimentary canal to the chain of ganglia. The respiratory 

 organs, however, whenever these are differentiated, are never 

 in the form of branchiae as in the Crustacea, but are in the 

 form either of pulmonary vesicles or sacs, or of ramified tubes, 

 formed by an involution of the integument, and fitted for 

 breathing air directly. Further, there are never "more than 

 four pairs of locomotive limbs, and the somites of the abdomen, 

 even when these are well developed, are never provided with 

 limbs ; " the reverse being the case amongst the Crustacea. 

 Lastly, " in the higher Arachnida, as in the higher Crustacea, 

 the 'body is composed of twenty somites, six of which are 

 allotted to the head ; but in the former class, one of the two 

 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 of 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 cephalothorax may be segmented (Solpugida) ; and the 

 abdomen may or may not be segmented. Though four pairs 

 of legs are present, the first is certainly homologous with the 

 labial palpi of the Insecta. 



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). 



