40 ME. M. LATJEIE ON THE 



Abdominal Appendages and Respiratory Organs. 



The full number of abdominal appendages (si.?) only persist as 

 sucb in Limulus. In this form they are somewhat modified, the 

 plates of each pair being connected in the middle line. That this 

 connection is secondary is evident from Kingsley's * figures, which 

 show the abdominal appendages quite distinct from each other in 

 early stages. The other aquatic forms, the Eurypterids, differ from 

 Limulus in the segmentation of the abdomen. The first appendage 

 is fused in the middle, and bears a well-developed median lobe, 

 which probably has some function in connection with repro- 

 duction, and is in some forms at any rate capable of partial 

 invagination. The second abdominal segment is covered by this 

 genital operculum and has no plate-like appendage, though it 

 bears a number of branchial lamellae. The third to sixth seg- 

 ments bear paired plates with branchial lamellse on their posterior 

 surfaces. The sternites persist in these segments ; at all events in 

 Slimonia — and this, one would expect, as a segmented abdomen 

 demands greater strength than one in which the segments are 

 fused together as they are in Limulus. 



I have stated elsewhere t what I believe to be the case as 

 regards the morphology of the anterior abdominal segments 

 in Scorpio and the Pedipalpi. To recapitulate briefly, tlie 

 Scorpions have all the segments well-developed, the second seg- 

 ment bearing the pectines, and the third to sixth having luug- 

 books. The genital plate is small and does not overlap the 

 second segment. In the Pedipalpi the genital plate covers two 

 segments as in Eurypterids, the second of which bears the first 

 pair of lung-books, which consequently lie under the genital 

 plate. The third segment is also covered by an appendage under 

 which lie the second pair of lung-books. I think the anatomy 

 and still more the development, as described above, fully bear 

 out this view. There can be no doubt that the first pair of lung- 

 books in the embryo Phrynus belong to the region covered by 

 the genital plate and not to the third segment. The first pair of 

 lung-books in the Pedipalpi thus correspond to the pectines of 

 Scorpions. Another difference between the lung-books of these 

 forms seems to be that in Scorpio tliey are formed, as I have 

 elsewhere maintained J, from paired appendages not united in 



* Journ. Morpb. vii. 



t Trans. E. S. Edinb. vol. xxxyii. 



+ Zool. Anz. 1892, no. 386. 



