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Key to Irish Chitons
Glossary


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 seperated) 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.