MORPHOLOGY OF THE PEDIPALPI. 35 



from the body-wall, the cavity of which contains at this stage a 

 certain number o£ mesoderm-cells. The hypodermis over the 

 greater part of it is very much like that o£ the rest of the body. 

 On the posterior surface, i. e. the surface nest to the body-wall, 

 however, the inner two-thirds is thickened, and the cells of the 

 thickened portion are beginning to arrange themselves in rows 

 more or less at right angles to the surface of the outgrowth. 

 This is the beginning of the lung-book. That this lung-book 

 belongs to the segment to which it is at this stage attached and 

 not to the one behind it is, I think, fairly certain. "With regard 

 to the first lung-book, which appears to be attached to the 

 posterior surface of the genital plate, it is not so evident to 

 which segment it belongs. The genital plate covers the ventral 

 surface of the first two segments, and the lung-book may either 

 be attached to the genital plate, and therefore belong morpho- 

 logically to the first segment, of which the genital plate is the 

 appendage, or it may be the sole survival of the appendage of the 

 second segment, which has otherwise entirely disappearad'. This 

 last I have suggested as being the case in the Eurypteridse * ; and 

 1 believe it to be the correct explanation in these forms also, but 

 only an examination of earlier stages can prove it. At all events, 

 it is pretty certain that the first lung-book belongs to segments 

 i. or ii., and not to segment iii. It is therefore not homologous 

 with the first lung-book of the Scorpion, which does belong to 

 segment iii., but is either the homologue of the pectines of the 

 Scorpion, i.e. appendage ii., or is a special structure, the appendage 

 of segment ii. having entirely vanished. The former is evidently 

 more probable a priori f. 



Of the development of the other organs I have not been able 

 to make out anything of importance. The whole of this paper is, 

 I feel, calculated rather to show what we may expect when the 

 embryology of this group is properly worked out than to say 

 what actually happens. If I have shown what important results 

 a study of these forms will almost certainly give us, and how 

 heavily handicapped any attempt to deal with the morphology of 

 the Arachnida must be until such a study has been made, I 

 have done all that I expected with the material at my disposal. 



* Trans. R. S. Edinb. vol. xxxrii. 



t A paper on the " Development of the Lungs in Spiders," by 0. L. Simmons 

 in the Am. Jonrn. Sci. Nat. for August 1S94, shows very similar structures, and 

 the author's com lusions agree for the most part with mine. 



3* 



