BIVALVES AND CUTTLES. 269 



is furnished in each valve with three projections, or teeth, 

 which lock into corresponding cavities, in much the same 

 way as that in which the bones of the skull in a mammal 

 are locked together. Further, the ligament in Tapes is 

 outside the shell, instead of being within it, and the two 

 muscle impressions are more distinctly marked than in the 

 mussel. 



Having by the study of the shell determined the position 

 of the closing muscles, kill your living specimens by dropping 

 them in hot water, slip a knife in between the valves, and 

 cut through the muscles as close to the shell as possible. 

 As soon as this is done, the elasticity of the ligament will 

 cause the shell to gape, and the upper valve can be gently 

 removed. If this be done carefully in both mussel and 

 Tapes, the animal in each case will be seen lying covered by 

 its mantle-flap, with the foot projecting more or less at one 

 end, and the apertures, or siphons, distinct at the other. 

 There is no head, but the mouth is placed at the opposite 

 end to the apertures, or siphons, and usually lies at the 

 more rounded end of the shell. It is immediately in front 

 of the foot, and has two little flaps, or palps, at either side. 

 On lifting up the mantle there will be found the plate-like 

 gills, of which a pair lie at either side of the foot. Pro- 

 jecting through the softer tissues will be also seen the firm 

 closing muscles cut through when the shell was opened, and 

 the foot, small in the mussel but large and distinct in Tapes, 

 as in most common Eivalves. 



"We cannot enter in detail into the anatomy of the 

 Bivalves they are difficult to dissect and to understand 

 but it may be well to explain briefly what structures vary 

 most frequently, and on what the usual classifications are 

 based. In the first place, there is much and very obvious 

 variation in the shell, in its shape, colour, and finer details. 

 Almost all early classifications depended on the shell. But 

 we have seen that the shell affords clear evidence as to one 

 structural characteristic of the living animal, the absence or 

 presence of siphons, so that a very early division is that into 

 siphonate and asiphonate forms. Again, in some cases, as 

 in Tapes, there are two distinct closing muscles, while in 

 others one only is present. As this character can also be 

 determined 'by the examination of the shell, it is very 



