SUBCLASS II 



EMBOLOBEANCHIATA 



787 



even ivhen external segmentation is subsequently lost. Anus on last abdominal segment. 

 Eyes, when "present, simple, variable in number. Respiration by lung-books or tracheae. 



The subclass of Embolobranchiata includes .spiders, scorpions, mites, ticks, etc., 

 and comprises in all thirteen orders. Of these four are entirely extinct, and of 

 those still living six have continued to exist since the Paleozoic, and only one is not 

 known to have fossil representatives. So far as the evidence of extinct forms goes, 

 the older members of the various orders seem to have resembled to a remarkable 

 degree those existing at the present day, and serve to illustrate the extreme antiquity 

 of living types of invertebrate animals. The majority of fossil sjiecies is known from 

 remains preserved in amber of Lower Oligocene age in eastern Prussia. The most 

 delicate jaarts, including the finest hairs and sjsiders' webs are to be found practically 

 unaltered within this transparent fossil resinous substance which exuded from ancient 

 Coniferous trees. 



The order Scorpionida {Scorpiones) is the oldest among the Embolobranchiata 



Fw. 1513. 



Upper Silurian primitive Scorpions, ralaeophonns. A, P. nuntlus Thor. and Lindst. Ludlow series 

 ^Clunian) ; Gotland. Restoration of dorsal aspect. % (after Pococlc). B, P. caledonicus Peacli. Ludlow 

 series (Clunian) ; Lanarkshire. Reconstruction of ventral aspect, in which the space for a genital operculum, 

 the pair of pectens, and the absence of any evidence of pulmonary stigmata are noticeable features, s/o (after 

 Pocock). 



and bears witness to a common origin with the Eurypterida. Scorjiions are 

 characterised by having tliree -jointed chelate chelicerae and six -jointed chelate 

 pedipalpi ; the head is fused with the thorax ; the abdomen is comj)Osed of twelve 

 segments, the last five of which are annular and form the so-called postabdomen or 

 tail. At the end of this latter is a telson modified as a poison gland with sting. 

 On the ventral surface is found a characteristic pair of appendages, the " comb." 

 Lung-books are present in four pairs. 



Silurian Scorpions are grouped together under the suborder Apoxtjpoda, which 

 contains a single family represented by the genera Palaeophonus Thorell (Fig. 1513) 



