MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 399 



the water the valves close and the water in the branchial chamber is discharged 

 principally throngh the hrancMal siphon, so that the membrane must have been raised 

 in order that this might occur. 



If the valves of the shell should suddenly close, much of the water in the branchial 

 chamber would escape ventrally. While this is going on, the siphons are being di-awn 

 within the shell, and of necessity against this pressure in the branchial chamber, 

 caused by the closing of the shell. If, instead of being closed, the branchial siphon 

 should be opened, and thus allow the imprisoned water to rush out, it would allow 

 their retraction to be more easily and quickly accomplished. 



The siphons of Venus, however, are small, and do not meet the difficulty in retrac- 

 tion encountered by the enormously developed siphons of My a arenaria, the "long- 

 necked clam," especially as the branchial chamber is here closed below over nearly its 

 entire extent, and allows little water to escape between the mantle edges. When the 

 siphons are contracted — and the process is always comparatively a very slow one — a 

 stream of water is discharged fi'om both, but mainly from the branchial. Though 

 finally brought Avithin the shell, their outer ends are somewhat exposed, as the shell 

 in their region is expanded and its valves can not meet behind them. The branchial 

 membrane is not here present, but may perhaps be represented by part of the thick- 

 ening in the partition between the siphons (Fig. 94, PI. xciii, hrm). If this is so, the 

 organ may have been lost because of its interference, though it may have been slight, 

 in the laborious process of withdrawing the siphons into the shell. 



Just what advantage may be subserved in the forms where the branchial mem- 

 brane is so greatly developed is not apparent to me. 



The series of figures 18 to 22 represents this i^osterior region of the body of Ven us 

 in vertical section. Fig. 18 has been cut just in front of the branchial membrane [br m ) 

 and the posterior adductor is seen above. The mantle edge has become muscular and 

 very thick at m, to form, farther back, the walls of the sii^hons. The bases of these 

 have been cut across in Fig. 19, and it may be here seen how their walls are gradually 

 constricted ofi:' from the mantle at x. A fold of the mantle (m) extends across under 

 the lower siphon and is also present in Fig. 20. In this latter figure the mantle fold 

 is entirely separated from the siphon walls, except dorsally, and these walls are seen to 

 be very thick and muscular, especially those of the cloacal sii^hon. In Fig. 21 they 

 have assumed a uniform thickness. In Fig. 22 the siphons are cut across where 

 they have protruded backward beyond the mantle edge. The basal part of the lumen 

 of the ui^per or cloacal siphon (Fig. 19, ns) is somewhat triangular in section, while 

 that of the branchial siphon is more nearly circular. Toward their outer end they 

 ai)pear as slits elongated dorso-ventrally, though not to so great an extent in the 

 living animal. 



The anatomy of the siphons of Mya is much the same as in Vemis. Fig. 29 is a 

 thick section just before the bases of the siphonal openings. The cloacal chamber is 

 cut across at cl, showing the gills at their posterior ends, separating the cloacal ft"om 

 the branchial chamber. The posterior end of the partition between the sijihons is 

 seen at hrm. The muscles, which are to become the siphon walls farther back, are 

 shown at ws. These, from this region to the ends of the siphons, are covered by a 

 thick, gelatinous, semitransparent tissue, ct. Still farther back, as in Fig. 30, in which 

 section the right side has been cut deeper than the left, are seen the siphon walls on 



