SÜBCLASS II 



EMBOLOBRANCHIATA 



787 



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

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



Tlie subclass of Embolobraiicliiata incliides spiders, scorj)ions, niites, ticks, etc., 

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

 tliose still living six liave continued to exist since tlie Paleozoic, and only one is not 

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

 tlie older menibers of tlie various Orders seem to have resembled to a remarkable 

 degree tliose existing at tlie present day, and serve to illiistrate the extreme antiquity 

 of living types of invertebrate animals. Tlie majority of fossil species is known from 

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

 delicate parts, including the finest hairs and sjDiders' webs are to be fonnd practically 

 iinaltered within this transparent fossil resinoiis siibstance which exiided from ancient 

 Cüiiiferous trees. 



The Order Scorpionlda (Scorpiones) is the oldest among the Embolobranchiata 



Fia. 1513. 



Upper Silurian pninitive Scorpions, l'alaeopTionus. A, P. nuntiils Thor, and Lindst. Ludlow series 

 (Clunian); Gotland. Hcstoration of dorsal aspect. ^/s (after Pocock). B, F. caleilonicus Peach. Ludlow 

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

 the pair of pectens, and the absence of any evidence of pulmonary Stigmata are noticeable features. »/o (after 

 Pocock). 



and bears witness to a common origin witli the Eurypterida. Scorpions are 

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

 pedipalpi ; the head is fused witli the thorax ; the abdomen is composed 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 witli sting. 

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

 Lung-books are present in four pairs. 



Silurian Scorpions ai'e grouped together uiider the suborder Apoxypoda, which 

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



