254 LECTURE XIX. 



altogether modified antennae, as Latreille supposed to be the case in 

 the present class. 



In the composition of the cephalothorax of spiders^ M. Audouin 

 has discovered that the tergal elements of the coalesced segments are 

 wanting, and that the back of the thorax is protected by the elon- 

 gation, convergence, and central confluence of the epimeral pieces ; 

 the sternal elements have coalesced into the broad plate (^fig, 114. A), 

 in the centre of the origins of the ambulatory legs, from which it is 

 separated by the episternal elements. The traces of the original se- 

 paration of the four epimeral pieces may be easily distinguished in 

 some spiders, as the 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 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 

 muscular chorion, the fibres of which surround the abdomen in 

 various directions. To the epiderm belong the hairy and spinous 

 appendages : the large bird-spiders (jMygale) are clothed with a thick 

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

 have complex hairs, like the down of birds, 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 Oxy- 

 opes are due to these scales. 



The muscular system is principally aggregated in the cephalo- 

 thorax for working the organs of mastication and locomotion ; 

 there are, however, special muscles in the slender jointed abdomen of 

 the scorpions for its inflection and extension, and more especially for 

 the purpose of wielding the poisonous weapon with which it is ter- 

 minated. 



The principal masses or ganglions of the nervous system are con- 

 centrated around the oesophagus in the cephalothorax of the scor- 

 pion. From the small supra-oesophageal or cephalic bilobed mass are 

 sent upwards the optic filaments, forwards the nerves of the forcipated 

 mandibles or " chelicera," and, backwards, the stomato-gastric 

 nerves ; the sub-oesophageal ganglionic columns distribute nerves to 



