VI 



FUSION OF MANTLE EDGES IN PELECYPODA 



173 



else become permanently 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 bran- 

 chial siphons are no more than prolongations of the mantle edges 

 on the posterior side into a tubular form. These ' siphons ' ex- 

 hibit the siphonal form more distinctly according as the adjacent 

 portions of the mantle become more definitely fused togetlier. 



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 Nucula^ Ar^ca, Anomia^ and 

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



D ^^ — -^ E i:o F 



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

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

 completely open ; C, mantle closed at one point ; D, mantle closed at two points, 

 with complete formation of siphonal apertures ; E, development of siphons, ven- 

 tral closure more extended ; F, mantle closed at three points, with fourth orifice : 

 /, foot; s.a, s.b, anal and branchial siphons; 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 wdiich 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 

 become 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 incur- 

 rent stream of water, the rudiments of the 'branchial siphon' 

 (Fig. 80, C). This is the case with most 3Iytilidae (see 

 Fig. 75) with Cardita^ Astarte^ and Pisidium. In the next 

 stage the branchial opening is separated off by the concrescence 



