Although the structured of the ctenidium is thus highly complicated in Anodonta, it is yet more so in some of the siphonate genera of Lamellibranchs.
2
In siphonate forms the pallial muscle is not simple, but is indented posteriorly by a sinus formed by the muscles which retract the siphons.
3
In this way the notches d, e of the hinder part of the mantle-skirt of Anodonta are in the siphonate forms converted into two separate holes, the edges of the mantle being elsewhere fused together along this hinder margin.
4
On the other hand, many which have a siphonate mantle-skirt are not provided with an introversible mouth-bearing al- B c..