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Anatomical Terms

Antemucronal area (fig. 1): the area on the tail valve (see fig. 9) which is located in front of the mucro and extending to the lateral edges of the valve.

Anterior (fig. 2): the front (head) end, or relatively nearer to the front.

Apex (pl. apices) (fig. 1): central posterior end of an intermediate valve; generally this point is somewhat projected, but the posterior valve margin may be straight (without an apex).

Articulamentum: underlying layer of the valves, generally smooth and pale-coloured. Lateral projections of the articulamentum form insertion plates. (The articulamentuum is not visible in living chitons)

Beak (fig. 1): a distinct projecting apex: central posterior end of an intermediate valve; generally this point is somewhat projected, but the posterior valve margin may be straight (without an apex)

Bifurcating (fig. 3): dividing into two branches; referring to patterns of sculpture on some chitons

Bristle (fig. 4): branches arising on setae; referring to specialised girdle covering carinate shaped liked a keel or ridge; referring to a raised edge down the central axis of an intermediate valve, creating a distinct ridge along the middle of a chiton.

Caudal sinus (fig. 1): an indented notch sometimes present on the posterior margin of the tail valve.

Central area (fig. 1): on each of the intermediate valves, the triangular area formed between the two lateral areas.

Diagonal rib (fig. 5): on the intermediate valves, a rib located at the anterior edge of the lateral area, extending from the apex to the margin of the valve. In some cases, the rib is not raised, but can be identified clearly by the different patterns of sculpture in the central area (anterior and ‘inside’ of the diagonal) and pleural areas (posterior and ‘outside’ of the diagonal) of the valve.

Dorsal (fig. 1): relating to or situated on the back; the upper surface of a chiton, which is mostly covered by the valves. The dorsal surface is the only surface normally visible in life, when chitons are attached to rocks.

Flammule: spot of colour resembling a small flame (may be reddish, tinged with red).

Foot (fig. 6): the strong muscular region occupying most of the ventral area

Girdle (fig. 1): flexible, leathery, muscular integument holding the chiton valves in place; often ornamented with scales or spicules (see fig. 4); also called the ‘perinotum’.

Granule (fig. 3): very small or minute elevation; referring to the sculpture on the dorsal surface of a valve.

Growth lines (fig. 3): concentric lines or ridges, visible on the dorsal surface of a valve, which correspond to the pattern of valve (shell) growth.

Hyaline: translucent or transparent.

Insertion plate (fig. 7): narrow marginal extension of the articulamentum of a valve which projects into the girdle.

Intersegmental: between segments (between valves).

‘Irregular’ scales (fig. 4): scales which are not arranged in a regular, overlapping pattern.

Jugal area (fig. 1): surface of the tegmentum in the middle of the dorsal surface of a valve; in some chitons, the central area of the valve is divided into a distinct jugal area surrounded by the pleural areas.

Jugum (fig. 1): longitudinal ridge, sharp or rounded, on intermediate chiton valves that are carinate, or a sub-area around the middle axis within the ‘central’ area of an intermediate valve.

Lateral area (fig. 1): side slope of a valve, posterior to the diagonal rib on an intermediate valve.

Lateropleural areas (fig. 1): an extended area of the dorsal valve surface combining the lateral areas and the central areas, where these areas are not distinctly divided by a diagonal.

Longitudinal (fig. 2): the ‘length’ of the shell from anterior to posterior; the directon of the longest axis on the whole animal.

Longtudinal rib (fig. 5): ribs that run parallel to the longitudinal axis of the body, pointing in the anterior-posterior direction.

Marginal fringe (fig. 4): spicules or spines arising from the edge of the girdle, in contact with the substrate that an animal is on, in life.

Microgranulose: covered with very small granules.

Mucro (fig. 1): a small, pointed process or part; referring to the distinctive small projection on the tail valve, which is usually close to the centre of the dorsal surface of the valve.

Nodulose: with small knobs or swellings.

Pallial fold (fig. 6): a fold of tissue lying between the foot and the girdle on the ventral side, forming the outside edge of the pallial groove.

Pallial groove (fig. 6): the space between the girdle and the foot on the ventral surface of a chiton that houses the gills.

Perinotum (fig. 1): flexible, leathery, muscular integument holding the chiton valves in place; often ornamented with scales or spicules (see fig. 4); also called the girdle.

Pleural area (fig. 1): on the intermediate valves, the area situated between the lateral area and the jugal area.

Posterior (fig. 2): the back (tail) end, or relatively nearer to the back.

Postmucronal area (fig. 1): the area of the tail valve posterior to (behind) the mucro, toward the outer valve margin.

Pustule (fig. 3): wart-like projection, distinctly larger (and usually more separated) than granules; referring to sculpture on the dorsal surface of the valves.

Quincunx (fig. 8): an arrangement of five things in a square, with one at each corner, and one in the center.

Radiating ribs (fig. 5): ribs which extend from the apex of a valve towards the anterior margin; often present on the head valve and the intermediate valves.

Rib (fig. 3): an elongated ridge; referring to sculpture on the dorsal surface of valves. At a microscopic level, ribs are often made up of granules in close patterns.

Riblet (fig. 3): small rib.

Scale (fig. 4): a small, flattened, rigid plate.

Sculpture (fig. 3): patterning on the tegmentum, where the surfaces is marked by raised on impressed small features.

Seta (pl. setae) (fig. 4): a projection on the girdle, somewhat flexible (and thicker than spicules or spines), often with bristles.

Spicule (fig. 4): small, slender, hard body, sharp-pointed, often needle-like; referring to projections that can cover the surface of the girdle.

Spine (fig. 4): pointed outgrowth which is stiff and sharp-pointed (larger and longer than spicules), usually solitary or in isolated tufts.

Sutural rib (fig. 3): a lateral rib lying along the posterior margin of a valve.

Suture (fig. 1): the lateral line or seam where two valves meet.

Tegmentum (fig. 7): the upper layer of the valves, not covered by the girdle (the only part of the shell valve visible in life).

Terminal (fig. 9): at the end; referring to the end valves (head and tail).

Transverse (fig. 2): lying across, running crosswise (from right to left); at right angles to the longitudinal axis.

Tubercle (fig. 3): a little knob or rough elevation.

Valve (fig. 9): one of the separable portions of a shell of a mollusc (the eight shells of a chiton, or equally the two shell halves of a clam).

Figure 1. Stylised illustration showing the names of shell (valve) parts in a chiton.

Figure 2. Dorsal view of a chiton; illustration showing orientation.



Figure 3. Stylised illustration showing different types of valve sculpture on chitons.




Figure 4. Stylised illustration showing different types of girdle covering in chitons




Figure 5. Stylised illustration showing different rib orientations in chitons.




Figure 6. Ventral view of a chiton; illustration showing the major anatomical features.




Figure 7. A single intermediate valve of a chiton.






Figure 8. Dots arranged quincunally (i.e. in quincunx)




Figure 9. The valves of a chiton

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