BIVALVE SHELLS-GENERAL STRUCTURE. 



619 



inhalent aperture, whilst the other (e) allows the fouled water to escape and is 

 termed the exhalent aperture. In the next stage the edges of the mantle are 

 united permanently at the point between these two openings, and in the 

 fourth at a second point below the inhalent aperture. In the fifth stage the 

 margins of these apertures have grown out into tubes (siphons), and the 

 remaining portions of the mantle margins have united all round, except 

 where the foot is protruded. Finally, the two siphons become united exter- 

 nally. In these cases there is frequently a fourth small aperture left in the 

 ventral margin. The siphons, the ends of which are frequently fringed, can 

 be wholly or partly withdrawn ; and it is to make room for them within the 

 shell that the pallial attachment is set further back at that point, giving rise 

 to the pallial sinus already spoken of. The proportionate length of the 

 siphons forms a rough index to the burrowing habits of the animal, since the 

 longer the siphons and the more complete the union of the mantle margins, 

 the deeper the mollusc can burrow into the mud or silt and still inhale clear 

 water from above. 



The gills lie underneath the mantle, one on either side, in the space 

 between the latter and the body of the animal. In structure these organs 

 vary progressively from a very simple to an extremely complicated form, and 

 since their features have been made the basis whereby this difficult group is 

 classified, it is necessary briefly to describe them. In the more primitive 

 bivalves the gill is of the foliobranch type (see ante, p. 614); but in the higher 

 Pelecypods they are filibranch, only the filiments of each of the two rows, 

 instead of running separate, have an arrangement whereby they interlock and 

 form a continuous membrane, like the web of a feather. The mechanism by 

 which this is brought about is extremely simple. At regular intervals on either 

 side of each filiment are little patches of stiff hairs which interlock with the 

 corresponding ones on the two neighbouring filiments, just as two brushes 

 can be made to do ; these are known as the ciliated junctions (Fig. 7 E). 

 Moreover, the two membranes lamellce thus arising do not simply depend 

 from the axis, but their edges are folded upwards that of the outer one 

 outwards, that of the inner, inwards, making a W in section as shown at 

 B, Fig. 7. 



Fig, 7. DIAGRAM ILI/CTBTRATING STTCCKSSIVB DEVELOPMENT OF PELECYPOD GILUJ. 



A, Protobranch (or Foliobranch). B, Filibranch. 



Oj Eulamellibranch. D, Septibranch. E, Diagram of Ciliated Junctions. 



A further complication of structure ensues when junctions, called inter- 

 lamellar junctions, are formed between the dependent part of each filament 

 and its reflected portion. In a more advanced stage still both the ciliated 



