I 



COMPAEATIVE MOEPHOLOGT OF THE GALEODID.E. 391 



constriction obliterated, and a diaphragm stretching all across the body. In this respect 

 Galeodes stands, perhaps, nearest the racial form. Where there is no waist or constriction, 

 it must he considered to have been secondarily obliterated. 



A structure showing such variations can, it seems to me, be deduced only by the infolding of the 

 membrane between two primitive undiflerentiated segments. It must date back to a time when the 6th 

 and 7th segments were joined by an undifferentiated intersegmental membrane capable of being drawn 

 in at any point or at all points. Evidence of this may be fonnd in the very simple character of the 

 intersegmental membranes between the segments of Scorpio or Galeodes immediately posterior to the 

 waist. This primitive condition, which was necessary to the foimation of a waist, is not supplied by 

 Limulus, in which the posterior region hinges in a rigid and highly specialized manner on the anterior, 

 ■or by the Eurypterids, where the segments of the tail-region almost certainly articulated with each 

 other, and with the head-region, for specialized motion in the sagittal plane for swimming. 



8. The Division of the Body into Eegions.— One of the earliest .specializations of the 

 primitive Arachnid was its division into two regions. The anterior, consisting of six 

 segments, was specialized for locomotion and prehension, and consequently sensation. 

 The posterior region, composed of tlie remaining segments, became a vegetative sac, 

 capable of great distension for the reception of large quantities of the liquid food sucked 

 in and of the genital products. The simjile imdifferentiated character of the abdominal 

 segments, and their different specializations of a varying munber of terminal segments 

 into tails, show that when the sac was first formed the segmentation was of the simplest 

 kind (PL XXIX. fig. 14). 



In this division into regions, the Arachnids resemble Limulus and the Eurypterids. In each case the 

 anterior region is locomotory and prehensile, and consists of six segments, while the posterior region 

 contains the remaining segments. Is this interesting agreement, however, any sign of close genetic 

 relationship ? Can the two regions in Limulus be the origin of the two regions in the Arachnids ? The 

 two regions in Limulus are clearly very pronounced specializations for certain purposes, but those 

 purposes are not the same as those which gave rise to the two regions in Arachnids, and this fact 

 is fatal to any close relationship between the two. The head-region in Limuhts is not only for locomo- 

 tion and prehension, hut it is also developed into a remarkable shield-like expansion not only for 

 protection but also for the reception of the digestive glands and of the genital products, which 

 thus crowd forward right to the anterior end of the body. The posterior region in Limulus consists 

 of the fused remains of a number of free flexible segments, once, no doubt, of great loocmotory 

 significance in swimming. In Limulus they persist now chiefly as carriers of gills, and in the 

 Eurypterids they served apparently as an organ of propulsion through the water. 



Admitting in theory that specializations fundamentally affecting the whole morphology of an animal 

 group may, after reaching an extreme, become very greatly modified towards an earlier and simpler 

 condition, I hold it to be impossible that any such specialization should be completely undone in order 

 to allow the animal to run its course a second time along a different line of specialization. And yet this 

 is what is implied if the specialized abdominal regions of Limulus or Euryplerus are to become the 

 abdominal regions of the Arachnids : they must have reverted to their primitive undifferentiated meta- 

 merism, for it is only from such undifferentiated segmentation that the abdominal regions of the different 

 Arachnids can be deduced. The full force of this argument will be further seen when (1) the limbs 

 and (2) the internal abdominal organs are appealed to. 



9. The Beak. — The possession of this organ in such diverse Arachnids as Galeodes, 

 Chernes, and Thelyphonns, and the easy deduction of the mouth-parts of Spiders, 



