110 ARACHNIDA. 



VIII. General Considerations regarding the Arachnida. 



In studying the Arachnida, the point of greatest importance and 

 interest consists in their relationship to those divisions of the 

 Arthropoda classed with them as Tracheata, i.e., the Myriopoda 

 and the Insecta. The Myriopoda, on account of their usually long 

 form of body and the slight differentiation of the different parts of 

 the body, demand less attention in this respect than the Hexapoda, 

 in which the very marked division of the body into three regions 

 calls for comparison with the segmentation of the Arachnida. In 

 such a comparison, however, a serious difficulty at once arises in the 

 different number of segments, and especially of limbs, found in 

 the two groups.* The fusion of segments which often takes place 

 among the Arachnida is of less consequence, since this may also occur 

 to a greater or lesser degree among the Insecta. The fusing of the 

 head and the thorax to form the cephalo-thorax must nevertheless 

 be emphasised as an important Arachnidan character. 



The Insects, as is well known, carry on the head a pair of 

 antennae, a pair of mandibles, and two pairs of maxillae, which, 

 on account of their structure and ontogeny, are justly regarded as 

 limbs ; further, there are three pairs of limbs on the thorax. The 

 Arachnida have only two pairs of cephalic limbs (the chelicerae and 

 the pedipalps), but four pairs of legs on the thorax. The attempts 

 which have been made to harmonise these differences are too 

 numerous to be treated here in detail. According to what may be 

 described as the prevailing view, there is no homologue in the 

 Arachnida for the antennae of the Insecta, but the chelicerae may 

 be homologisecl with the mandibles, the pedipalps with the first 

 maxillae, and the four ambulatory limbs with the second maxillae 

 and the limbs that follow them. The chelicerae have, however, by 

 some been considered to correspond to the antennae. "We are not 

 disposed to accept either of these views, but, for reasons to be given 

 later, compare the chelicerae to the second antennae of the Crustacea, 

 for which a homologue is wanting in the Insecta. The first antennae 

 of the Crustacea, which correspond to the antennae of the 

 Insecta, are not present in the Arachnida. The pedipalps can at 

 once be homologised with the mandibles of the Insecta (and 

 Crustacea), the four pairs of ambulatory limbs with the two pairs 

 of maxillae and the legs of the Insecta, but in this case one pair of 

 thoracic extremities is wanting in the Arachnida. This, however, 



* [On this whole discussion compare Editorial note, p. 117.] 



