448 LECTURE XIX. 



from the thorax, and it suppoi'ts the first of the four pairs of legs 

 usually ascribed to the Arachnids. These modifications, with the 

 union of the ocelli into two groups, indicate the Galeodes to form the 

 passage to the Hexapod insects. 



The modification and the connections of the pair of appendages 

 which succeeds the maxillary palpi in the Galeodes demonstrate 

 that they are the homologues of the labial palpi in Hexapod insects. 

 In most spiders they are explorers, and are carried forwards ex- 

 tended, while the other legs are used in progression. The position 

 of the rudimental antennae in the same interesting genus confirms the 

 indication afforded by the nervous system in spiders and scorpions, 

 that the antennae are confluent with the so called mandibles, if these 

 be not altogether modified antennae. They are not the homologues 

 of the mandibles of insects. 



In 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, convergence, and central 

 confluence of the epimeral pieces ; the sternal elements have coalesced 

 into the broad plate (^fig- 172, K) in the centre of the origins of the 

 ambulatory legs, from which it is separated by the episternal 

 elements. The traces of the original separation of the four epimeral 

 pieces may be easily distinguished in some spiders, as in Pholcus 

 rivulatus. The non-development of the tergal elements explains the 

 absence of wings, which we have seen, in the Articulata, to be the 

 appendages of those elements, and to be very frequently restricted to 

 the branchial function. The tergal parts of the thoracic segments 

 are equally absent in the decapod Crustacea ; but, in them, the back 

 of that division of the body is protected by the carapace, continued 

 backwards from certain cephalic segments : when this is raised, the 

 epimeral pieces are seen converging, as in spiders, but not meeting 

 and coalescing. 



The soft and flexible integument of the abdomen in mites and 

 spiders gives no indication of the segments or their component parts, 

 but it is favourable for the study of its intimate organisation. Beneath 

 the epiderm and pigmental layer may be distinguished a thin chorion, 

 the fibres of which, probably contractile, surround the abdomen in 

 various directions. To the epiderm belong tlie hairy and spinous 

 appendages : the large bird-spiders {Mygale) are clothed with a thick 

 coat of hair : some of the smaller species, as Aranea domestica, 

 have complex hairs, like the down of bii'ds, implanted by a stem. In 

 other spiders, similarly implanted stems support scales, analogous to 

 those of the Lepidoptera ; the bright colours of the Salticce and 

 Oxf/o])es are due to these scales. 



