THE RESPIRATORY ORGANS. 



77 



(No. 61) points out in addition to this that the rudiment of the lungs in the 

 Araneae so closely resembles that of the gills in Limulus, that the one might 

 be mistaken for the other. Further, the rudiments of the gills in Limulus are 

 somewhat sunk below the level of the ventral surface. The position of the 

 develojiing lung at the posterior side of the limb is also of great importance. 

 We are therefore disposed to regard the lungs of the Arachnids as modifications 

 of once functional gills (Vol. ii., p. 35S). When the gills are drawn into the body 

 in such a way that the free posterior edge of the limbs, by its partial fusion with 

 the body-wall, yields the stigma, the lower (true anterior) surface of the limb which 

 lies close to the body must become the ventral body-wall of that region. This 

 is in agreement with MORIN's statement, according to which the disappearing 



fit 



-^ $— 3 ITT 



°' ^ ««° ^^^^y»a gCT ^ in u w^^^.^ ^ 



L * s m jBiBffnM— ■■— B M|^[M ■IIIIIIPMpiii ihiiiii i mu m 



Fig. 40. — A somewhat diagrammatic longitudinal section through an Araneid lung (after 

 MacLeod). W, lung-leaves : ch, chitiuous covering of the body, and beneath it the cells 

 of the hypodennis ; d, dorsal side ; dk, dorsal air-chamber ; do, dorsal surface of a lung-leaf, 

 provided with a thicker layer of chitin and teeth ; /, fibres of connective tissue attached to 

 the lung-sac; h, posterior side; Ik, air-chambers; Ir, common air-space of the lung-sac; 

 st, stigmatic aperture ; v, ventral side ; re, ventral surface of a lung-leaf with thinner and 

 more even layer of chitin ; vo, anterior side; w, posterior wall of the lung-sac, with the 

 cellular matrix. Between the two lamellae of each lung-leaf (fil) the (darkly shaded) 

 transverse trabeculae can be seen. 



limb yields the outer covering of the lung. In such an origin of the lungs, it is 

 clear that the lung-leaves arise chiefly from the ventral wall of the sac (posterior 

 surface of the limb, Fig. 40). The lung-leaves correspond directly to the leaves 

 of the gills which are still found in Limulus. We therefore refer the lung-leaves 

 to-the leaves of the sills, without assuming an invagination of the latter such as 

 has been sometimes demanded (Vol. ii., p. 359). It has been stated that in such 

 a phylogenetic origin of the lung-book we should expect to see the leaves 

 appearing as projecting folds on the abdominal limbs, before the depression 

 occurs, so that the lung-book would pass ontogenetically through a gill-stage. 

 Such an ontogenetic stage does not occur, but the invagination takes place first, 

 and the folds then form within it. It ajipears to us to be demanding too much 

 to expect to find such an ontogenetic stage, and to be going too far to found an 



