Definition
The two most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. The most abundant metallic element in molluscs is calcium.
Molluscs have developed such a varied range of body structures, it is difficult to find synapomorphies (defining characteristics) to apply to all modern groups. The most general characteristic of molluscs is they are unsegmented and bilaterally symmetrical. The following are present in all modern molluscs:
- The dorsal part of the body wall is a mantle (or pallium) which secretes calcareous spicules, plates or shells. It overlaps the body with enough spare room to form a mantle cavity.
- The anus and genitals open into the mantle cavity.
- There are two pairs of main nerve cords.
Other characteristics that commonly appear in textbooks have significant exceptions:
| Class | |||||||
| Characteristic | Aplacophora | Polyplacophora | Monoplacophora | Gastropoda | Cephalopoda | Bivalvia | Scaphopoda |
|---|---|---|---|---|---|---|---|
| Radula, a rasping "tongue" with chitinous teeth | Absent in 20% of Neomeniomorpha | Yes | Yes | Yes | Yes | No | Internal, cannot extend beyond body |
| Broad, muscular foot | Reduced or absent | Yes | Yes | Yes | Modified into arms | Yes | Small, only at "front" end |
| Dorsal concentration of internal organs (visceral mass) | Not obvious | Yes | Yes | Yes | Yes | Yes | Yes |
| Large digestive ceca | No ceca in some aplacophora | Yes | Yes | Yes | Yes | Yes | No |
| Large complex metanephridia ("kidneys") | None | Yes | Yes | Yes | Yes | Yes | Small, simple |
Read more about this topic: Mollusca
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