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 
withdrawn. 
The gills lie underneath the mantle, one on either 
side (Plate XV., Fig. 3, portion shaded with straight 
lines ; and Fig. 8, bv), 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, a); 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- 
