392 MR. H. M. BERXAED OX THE 



Scorpio and Phrynns, from such an organ, render it almost certain that a beak was 

 present in the original Arachnid. The construction of the heak out of a lahrum or 

 prostomium in its original anterior position, and of a labium formed l)y the protrusion of 

 the ventral surface of the 1st segment, seem necessarily to imply an undifferentiated 

 condition of tbe anterior segmentation in the immediate ancestors of the primitive 

 Arachnid. The beak points back to a simple Annelidan condition of the segmentation 

 (PI. XXXIV. fig. 17 A). 



This, to my mind, renders the derivation of the Arachnids from Limulus or Euryjitertts, with their 

 extremely specialized anterior segmentation, well-nigh impossible. It seems to me hardly possible that 

 any Limulus-Wke animal with the ventral position of the prostomium (PI. XXXIV. fig. 17 B) and the 

 obliteration of the steruites {cf. supra) could ever again recover the anterior position of labrumand labium 

 with so little sign of previous modification. What direct evidence we have as to the recovery of an 

 anterior position of the mouth from the ventral position of Apiis and Limulus shows that this is attained 

 by the rudimeutation of the prostomium or labrum. We have in Limulus a slight tendency to move 

 forward from the position of the labrum in the Trilobites, owing to the crowding round the mouth of 

 so many jaws, the consequence being that, as compared with the labrum of the Trilobites, it is a very 

 rudimentary organ. And further, in Eurypterids it appears completely to have vanished, or at least is not 

 discoverable. After having once taken up the ventral backward position, it could not so completely undo 

 that specialization as to tilt forward so as to point anteriorly as it is found to do in the Arachnids, leaving 

 no traces whatever of its former structure and position. The labrum of the Arachnids occupies very 

 nearly the same position it originally held in the Annelidan ancestor, and there is no sign that it has ever 

 occupied any other. 



10. The Ocular Tubercle. — An ocular tubercle, ^. e. an island of the original dorsal 

 surface left between the cephalic lol:)es as they approached and fused in the middle line, 

 must have been present in the primitive form. It is now very largely obliterated, but 

 persists in Galeodes and the Aviculariidoe and in some Scorpions, in which latter its 

 origin is most clearly shown. It is quite peculiar to the Arachnids as a class. 



This ocular tubercle is unknown in any other Arthropods, as, indeed, we might expect, since no other 

 Arthropod luiderwent the same peculiar distortion of the primitive segments. Limulus and the 

 Eurvpterids naturally show no traces of such a structure, whereas were they primitive Arachnids we 

 should expect to find it in its least difl'erentiated condition, somewhat as we find it in the Silurian 

 Scorpion Palmophonus [cf. text & PI. XXVII. fig. 10, ds). 



These several points so far disciissed deal entirely with the external specialization of 

 the primitive segments. They sejoarate the Arachnids comj)letely from all otlier Arthro- 

 pods, and furthest of all from Limulus, whose essential morphology, or, in other words, 

 whose early differentiation of the primitive ancestral metamerism, was the very reverse of 

 that of the Arachnids. As Arthropods, no relation whatever exists between them; as 

 segmented animals, however, they are both derivatives from the Chaetopod Annelids, but 

 along diflferent and opposite lines of sjiecialization. 



11. The First Appendage. — The chelicerse of the Arachnids can be most easily deduced 

 from a claw-like limb of three joints, from which both the existing forms of the 

 appendage can be deduced. 



It is not easy to deduce the primitive claw-form of the limb from an ancestor already equipped with 

 specialized pincers, such as those of Limulus. 



