THE GILLS. 



69 



The papillae are thickened at their free ends (Fig. 29 ^4). The 

 continued extension, anteriorly and posteriorly, of these free ends 

 leads to fusion of the papillae, so that the series ma} 7 now be regarded 

 as a membrane perforated by parallel vertical slits, this membrane 

 representing the rudiment of the inner branchial leaf. In most 

 Lamellibranchs, however, each leaf consists of two lamellae. The 

 second or ascending lamella of the inner leaf arises by the bending 

 inward of the free edge of the primary fold formed by the fusion of 

 the papillae (Fig. 29 J5) ; this new lamella then grows upward parallel 

 to the (now outer or descending) lamella towards the base of the 

 latter. The inner lamella thus formed is at first an unbroken mem- 

 brane, the slits only appearing in it when it has increased in size. 



KHJ. 29. Diagram of the development of the gills in a Lamellibranch possessing two 

 branchial leaves on each side. /, inner, e, outer branchial leaf; /, foot ; in, mantle. 



The outer branchial leaf now appears and becomes applied to the 

 posterior half of the base of the inner leaf when the latter consists 

 of about twenty papillae and when its inner or ascending lamella is 

 partly formed (Fig. 29 C). The outer leaf forms on the whole in 

 the same way as the inner, but, in it, papillae are said to form 

 anteriorly as well as posteriorly, and the leaf, in order to yield a 

 second lamella, bends outwards and not inwards (Fig. 29 D). The 

 fusions of the free edge of the inner lamella of the inner leaf and the 

 outer lamella of the outer leaf with the integument of the body take 

 place later, and vary in extent greatly in different Lamellibranchs, 

 being altogether wanting in some. 



