FUSION OF MANTLE EDGES IN PELECYPODA 



173 



united at one or more points. This fusion of the mantle edges 

 corresponds to important changes in the organisation of the 

 animal as a whole. The anal and branchial siphons are no more 

 than prolongations of the mantle edges on the posterior side into 

 a tubular form. These ' siphons ' exhibit the siphonal form more 

 distinctly according as the adjacent portions of the mantle become 

 more definitely fused together. 



This progressive fusion of the mantle edges may be taken as 

 indicating definite stages in the development of the Pelecypoda. 

 A perfectly free mantle edge, joined at no point with the edge 

 of the adjacent mantle, occurs in Nitcula, Area, Anomia, and 

 Trigonia (see Fig. 80, A, B). Here there is nothing in the 



Fig. 80. — Diagram illustrating the various stages in tlie closing of the mantle in Pelecy- 

 poda : A, mantle completely open ; B, rudiments of siphons, mantle still completely 

 open ; C, mantle closed at one point ; D, mantle closed at two i^oints, with com- 

 plete formation of siphonal apertures ; E, development of siphons, ventral closure 

 more extended ; F, mantle closed at three points, with fourth orifice : /, foot ; s.a, 

 s.b, anal and branchial sijihons ; 1, 2, 3, first, second, and third points of closure of 

 mantle. (After A. Lang.) 



nature of a siphon, either anal or branchial ; in other words, no 

 contrivance exists to prevent the spent water which has passed 

 over the branchiae from becoming mixed with the fresh water 

 which is to reach them. When the mantle edges are fused at 

 one point only, this is invariably on the middle part of the 

 posterior side, thus separating off an anal opening which may 

 Ijecome prolonged into a tube-like form. At the same time the 

 adjacent underlying portions of the mantle edges draw together, 

 without actually coalescing, to form an opening for the incurrent 

 stream of water, the rudiments of the 'branchial siphon' (Fig. 80,C). 

 This is the case with most Mytilidae (see Fig. 75) with Car- 

 dita, Astarte, and Fisidiuvt. In the next stage the branchial 



