246 LECTURES ON MOLLUSCA. 



gantl} 1- fringed with feelers to aid the currents. As the water is sucked 

 into the gill-chamber, the plates collect the minute plants and animals 

 that float in it. These lie in their grooves, and are gradually formed 

 into threads, which are carried down towards the mouth. Here they 

 are laid hold of by a pair of long delicate flaps or lips, which draw the 

 threads to the mouth. The filtered and carbonized water is forced 

 back, along with the foecal matter, through the excurrent pipe, which 

 is generally longer than the other, in order not to interfere with the 

 purity of the inhaled current. These mollusks generally live covered 

 up with sand or mud ; and might escape detection, but for the slight 

 protrusion of their pipes ; yet the disturbance they make in the water 

 by their vigorous breathing is well known to all keepers of aqua- 

 riums. 



The bivalve shells are objects of great beauty, both as respects form, 

 sculpture, and color. It is however unfortunate for geological purpo- 

 ses that the principal differences among them depend on the internal 

 structure, the hinge teeth, the muscular impressions, and the marks 

 of the siphon pipes, which cannot often be seen in fossil specimens. 

 Dr. W. B. Carpenter has however shown, (v. Reports of the British 

 Association, 1844, pp. 1-24,) that the structure of the shell affords 

 very characteristic marks in several of the families and genera -, by 

 which the affinities of fossil specimens and even fragments may often 

 be satisfactorily determined. 



The bivalves do not group themselves into natural orders like the 

 univalve mollusks. There is a much greater similarity of type among 

 them, and the points of difference are not constant among the creatures 

 whose general relationships correspond. If we compare a "clam" 

 with an oyster, we see at once that the clam has two water pipes, a 

 foot, and the mantle closed in front ; while the oyster has an open 

 mantle, without foot or pipes, and has only one muscle instead of two 

 to work the valves. Yet if we separate according to any one of these 

 characters, the division, will not suit others, and we shall be obliged 

 to part closely allied groups. It may be best therefore to allow the 

 families to follow each other in a natural order, without insisting on 

 orderly or suborderly lines of demarcation. The following are how- 

 ever the leading types of structure : 



I. Borers, Razor-shells, Mya-clams, &c, in which there are two 

 long water pipes, more or less united and retractile., the gills being- 

 produced into the breathing pipe, and the mantle closed except for the 

 foot and pipes. 



II. Venus-clams, Tellens, Cockles, &c, in which the pipes are gen- 

 erally separate, the gills not produced, and the foot mostly flattened 

 for crawling or leaping. 



III. Sea and Freshiuater Mussels, &c, in which the mantle-lobes are 

 only closed to form a breathing hole. 



IV. Oysters, Fan-shells and Arks, in which the mantle-sides are 

 entirely separate. 



The Venus tribe may be considered as the typical and most highly- 

 organized Lamellibranchs ; from these the stream of affinities flows 

 down through the Mussels and Oysters, towards the Palliobranchs ; 

 and through the Borers towards the Tunicaries. As however we can- 



