28 THE LIFE OF THE MOLLUSCA 



fourth stage (D), at a second point below the inhalent 

 aperture. In the fifth stage (E), the margins of these 

 apertures have grown out into tubes ("siphons," 

 Plate XV., Fig. 7), and the remaining portions of the 

 mantle margins have united all round, except where 

 the foot is protruded. Finally (F), the two siphons 

 become united externally (Plate XV., Fig. 4). In 

 these cases there is frequently a fourth small aperture 

 left in the ventral margin. The siphons, the ends of 

 which are often fringed, can be wholly or partly 

 withdrav/n. 



The gills lie underneath the mantle, one on either 

 side (Plate XV., Fig. 3, portion shaded with straight 

 lines ; and Fig. 8, br), in the space between the latter 

 and the body of the animal. These organs vary 

 progressively from a very simple structure to a very 

 complicated one, and since their structure has been 

 made the basis of the classification of the group, a 

 brief description of it is necessary. In the more 

 primitive Bivalves the gill is of the aspidobranch 

 type {ante, p. 7, Plate IV., Fig. 5, fl) ; but in the 

 higher Pelecypods they are filibranch, only the fila- 

 ments of each of the two rows, instead of remaining 

 separate, have an arrangement whereby they inter- 

 lock 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 filament are little 

 patches of stiff hairs which interlock with the corre- 



