310 ME. H. M. BERNAED ON THE 



which is a transverse section, shows that posteriorly they are a pair of blind pockets 

 restino- on the dorsal surface. These cephalic lobes may then be explained as the lateral 

 re^-ions of the first segment which, for tlie attachment of the muscles, have travelled 

 upward and backward in the process of tilting forward the powerful chelicerae (as shown 

 in the diagram, PI. XXVII. fig. 8) into the striking position which they occupy dorsally 

 to the mouth and labrum. The position of the chelicerge is one of the features which 

 distinguish the Arachnids from all other Arthropods. If the above explanation of the 

 origin of these lobes is correct, we ought, then, to find them, or traces of them, in all 

 other Arachnids. 



I have already pointed out (9) the marked likeness between these lobes in Galeodes 

 and the cephalic lobes which appear in the embryos of Spiders. Further, by taking 

 account of yolk-displacement, the embryonic procephalic lobes of the Scorpionidse may 

 be referred to the same structures. 



We have, then, to explain the embryonic cephalic lobes of Arachnids as recording the 

 movement of the basal regions of the chelicerse (which, from being postoral limbs, became 

 preoral) laterally round and above the mouth till they met in the dorsal middle line 

 behind the labrum and the eyes. 



The only other Arachuid with the cephalic lobes as pronounced as they are in Galeodes is Schizonotus, 

 which, as we have seen, resembles Galeodes in the non-fusion of the last two (V three) cepbalothoracic 

 segments. In the majority of Arachnids, the cephalic lobes have been more or less secondarily 

 obscured. The two chief causes of this gradual obscuration of the cephalic lobes in the Arachnida are 

 (1) their complete fusion with the terga of the 4th, 5th, and 6tb cepbalothoracic segments, so that the 

 dorsal surface is covered by a single piece ; (2) the secondarily acquired enormous development of the 

 pedipalps as compared with the chelicerae, so that the cephalic lobes, which were primarily the carriers of 

 the originally powerful chelicerse, are no longer so pronounced as to be immediately recognizable [cf. 

 fig. 9). Many Arachnids still, however, show unmistakable traces of the paired lobes divided by the 

 median suture. In some Scorpions, the formation of the carapace out of a pair of median anterior 

 plates with the triangular remains of the tergum of the 3rd segment, followed by three distinct terga, 

 can still be made out. In Thelyphonus the lobes can still be recognized. In the Araneaj, except in 

 the Aviculariidse, which are in other respects also primitive, they are bent down anteriorly to form a 

 " forehead,"' and are still here and there traceable on the carapace, e. g. of the Drassidse. 



The Ocular Tubercle. — If this view of the secondary origin of the anterior dorsal 

 surface is correct, il is clear that, if the eyes were primitive structures belonging to the 

 primitive dorsal surface, the cuticle immediately round such eyes must be part of that 

 original dorsal surface, and not primarily continuous with that of the new dorsal surface, 

 formed by the meeting of the cephalic lobes in the dorsal middle line. We find some 

 evidence that this is indeed the case. The ocular tubercle in very many Galeodidse 

 obviously protrudes from between the median suture, and in some species of RJiax it 

 often differs markedly in colour, being bright green, while the surrounding surface 

 is red or reddish brown, and in sections there are signs that the eye-tubercle is a 

 discontinuous element in the cuticle {ot, PI. XXXI. fig. 1). 



In some other existing Arachnids, the median eyes show signs of not belonging to the surrounding 

 cuticle. In Scorpio they appear to be protruding through a median suture, an impression which is some- 

 what increased on dissection. We have, however, only to look at the drawing of the Silurian Patau- 



