ARACHNIDA. 



573 



M 



finely punctate or minutely porous structure, along with numerous 

 oval pits of different sizes, each of which exhibits a central dark 

 spot or ring. These latter are not perforations, 'but are the sockets 

 of hairs. The skin of a fossil 



Scorpion from the Carbonif- '\ : J*?%& * M j 



erous rocks (collected by the : . 



Geological Survey of Scot- , 



land) also exhibits a minutely ; 



reticulate or porous structure, „ " 



and presents larger oval pits, - , 



which show a central dark : \. V" ' ..,..'/ , . ' ;) 



spot, and which seem to be \\ b 



quite similar to the hair- 

 sockets of the recent Scor- 

 pions (fig. 430, c and d). 

 There are also still larger 

 oval perforations, which are 

 surrounded by a fibrous ring, 

 and which are true pe?-fora- 

 tions, passing completely 

 through the thin integument. 

 These may represent the 

 sockets of large bristles, from 

 which the delicate chitinous 

 membrane forming the origi- 

 nal floor of the socket (traces 

 of which are still sometimes 

 visible) has disappeared ; or 



they may be truly pores. In the latter case, they do not appear to 

 have any representative in the integument of the recent Scorpions. 



The segments of the head and thorax in the Arachnida are gen- 

 erally amalgamated to form a " cephalothorax," which is in some 

 cases (Solpugidce) segmented. The upper surface of the head carries 

 the eyes, which are always simple and are never supported upon 

 movable peduncles. There are no proper " antennae," but the place 

 of these is taken by a pair of prehensile organs known as the 

 " chelicerae," " falces," or " mandibles " in different groups of the 

 class (fig. 431, c). There is a single pair of "maxillae" or proper 

 jaws, which carry long jointed appendages — the " maxillary palpi." 

 The palpi are sometimes leg-like in form (" pedipalpi "), or they 

 may, as in the Scorpions, be converted into nipping-claws (fig. 

 431, m). Immediately posterior to the mouth is a lower lip or 

 " labium," which is unpaired in the Spiders, but in some cases is 

 partially divided by a longitudinal groove, and is thus seen to repre- 

 sent a second pair of maxillae. The four segments of the thorax 



Fig. 430. — A, Exoskeleton of a recent Scorpion 

 enlarged, and (b) still more highly magnified, show- 

 ing minute pores together with larger pits to which 

 hairs were attached, c, Portion of the skin of a 

 fossil Scorpion from the Carboniferous rocks of Scot- 

 land, enlarged, showing pits for the attachment 

 of hairs. The large oval perforations probably also 

 represent the sockets of hairs, of larger size. d, 

 Portion of the same enlarged further, showing a 

 finely reticulate or porous structure. (Original.) 



