3. Cephalopoda.
a. Tetrabranchiata.
b. Dibranchiata.
4. Polyplacophora.
5. Scaphopoda.
II. LAMELLIBRANCHIATA.
a. Dimya.
b. Monomya.
[100] The reader is referred for the segmentation to pp. [98‑102], and to the special description of separate types.
[101] Rabl (No. [268]) describes a blastopore of this form in Planorbis which closes at the mouth.