103 



By what steps this change takes place the material at my disposal does 

 not show. 



It seems to me that this embryonic position of the lamellae is cal- 

 culated to throw some light on the probable mode of derivation of the 

 lung-books of terrestrial Arachnids from the gills of an aquatic an- 

 cestral form. At present two theories have been put forward on this 

 point, those namely of Lankester andMacleod. 



According to Professor Lankester's theory^ the lung-books 

 have been formed by the invagination of an abdominal appendage in- 

 termediate in form between the gills of Limulus and the pectens of 

 Scorpio. That such an invagination may take place, and take place as 

 Professor Lankester suggested as an adaptation to the conditions of 

 embryonic life, has been shown byE.eichenbach who describes the 

 last pair of abdominal appendages in Astacus as forming in this way 

 in the embryo and being evaginated later. There is also nothing in 

 the structure of the adult lung-book to preclude such a mode of deri- 

 vation. If, however, the embryonic position of the lamellae represents, 

 as I believe it to do , a stage in the phylogenetic history of the race, 

 the probability of Professor Lankester's theory is, I think, conside- 

 rably diminished, and for the following reason: If we imagine an 

 appendage, bearing a series of lamellae along one side, to be invagina- 

 ted , so as to form a structure resembling a lung-book , then the stig- 

 matic aperture and air space would bear the same relation to the 

 lamellae which the axis of the appendage bore previous to invagination. 



Fig. 3. 

 J. 



Fig. 3 und 4. Diagrammatic representation of the effects of invagination of 

 an appendage with lamellae attached to the posterior side. 



For example, if the lamellae were borne on the posterior margin of 

 the axis of the original appendage then after invagination they would 

 lie on the posterior side of the stigma and air space. A glance at 

 •figs. 3 and 4 will, I think, make this point clear. This being so, the 

 appendage, from the invagination of which the arrangement shown in 



2 a. J. M. S. Vol. XXV. p. 339. 



