Taxonomy of Shellfish

Unit 10 - Taxonomy of Phylum Mollusca

10.3. Class 2. Amphineura

In Greek, Amphi means both and Neuron means nerve. These are the most primitive molluscs.

Key characters

• Body either elongated and vermiform or dorso-ventrally flattened.
• Head indistinct, without eyes and tentacles.
• Foot broad, flat, sole-like and ventral in position.
• Shell and mantle covering the body from dorsal and lateral sides.
• 6-60 pairs of gills situated laterally in the pallial cavity on either side of foot, sometimes they may be reduced to a pair.
• Mouth and anus at the opposite ends of body.

Class Amphineura is divided into two orders,

Order-1. Aplacophora

• The body long, narrow, worm like, shell absent.
• Mantle contains tiny calcareous spicules.
• Foot reduced or absent
• Gills absent or reduced to a pair located in the cloacal cavity.

Eg. Chaetoderma, Neomenia

Order-2. Polyplacophora

• Body elliptical, convex dorsally and flattened ventrally.
• Shell dorsal and formed of eight calcareous pieces arranged in a longitudinal row.
• Mantle with spicules disposed along the margins.
• Gills 6 to 8 pairs, present in the pallial groove on the lateral sides of foot.
• Foot greatly expanded, forming a large flattened sole, used for locomotion and for maintaining firm contact with the rock surface.
• Head without eyes and tentacles.
• Well developed radula.

Eg. Chiton