ANATOMY AND EELATIONS OF THE EURYPTERIDS. 525 



in the middle line. Another point of great morphological importance is the fact that 

 Limulus has a pair of plate-like appendages on the second abdominal segment. This 

 would seem to indicate that Limulus branched off from the Eurypterid stem before the 

 genital operculum was so highly specialised as it is in the Eurypterids, and, consequently, 

 before the appendage of the second abdominal segment had become reduced. This is also 

 hinted at by the absence in Limulus of anything comparable to the central lobe of the 

 genital operculum. Probably also the metastoma had not reached the high state of 

 development it has in Eurypterids, and the last pair of legs was less highly specialised. 



Limulus, then, represents a more primitive type in almost every respect except the 

 fusion of the abdominal segments, and is to be related to the Eurypterids not by direct 

 descent, but through a comparatively unspecialised ancestor. Whether this ancestor is 

 one of the Trilobites must remain a matter for speculation, but it seems within the bounds 

 of possibility. 



Scorpio. 



The relationship of Scorpio to Limulus has long been maintained, and no one who has 

 studied the articles by Lankester, Claus, &c, on the subject will have much doubt that 

 it exists. The relationship of Scorpio to the Eurypterids, however, has never been so 

 fully dealt with. Lankester points out certain characters which they have in common, 

 such as the number of body segments, thoracic appendages, &c, and these seem to show 

 that the point of divergence of Limulus, Scorpio, and Eurypterids must have been before 

 the fusion of the body segments which we have in Limulus. On the other hand, the 

 second abdominal segment in Scorpio is well developed, and shows no signs of having 

 been suppressed by the genital operculum. This would seem to give us a clue to where- 

 abouts the Scorpions came off from the Eurypterid stem, namely, before the great 

 development of the genital operculum. The lung-books on the third to sixth abdominal 

 segments are probably derived, as I have pointed out elsewhere,* by the fusion of the 

 margins of the abdominal appendages to the ventral surface of the body, much in the way 

 suggested by MACLEOD.t My reasons for holding this view rather than that of Professor 

 Lankester,| wno nas suggested that lamelligerous appendages became invaginated are, 

 briefly, (l) that to produce the structure shown in the lung-books of young Scorpions, the 

 appendage prior to invagination would have had to have the branchial lamellae attached 

 to its anterior surface — a position in which they are not found either in Limulus or 

 Eurypterids ; and (2) that one can see how the transformation could take place, step by 

 step, as the animal became more terrestrial in its habits, instead of having to explain it 

 by a sudden change in the embryonic development, as Professor Lankester's hypothesis 

 demands. 



* Zool. Anz., No. 386, 1892. t Arch, de Biol, vol. v. J Q. J. M. S., vol. xxv. 



VOL. XXXVII. PART II. (NO. 24). 4 K 



