crinoid.art

October 28th, 2005

INTRODUCING CRINOID PARTS

Most
of a crinoid’s body, in fact usually at least 80% or so, consists of a
skeleton of calcium carbonate pieces, or ossicles, held together
by ligaments and muscles. This skeleton explains both why crinoids make
good fossils (calcium carbonate is basically limestone) and why not too
many creatures subsist on a crinoid diet (they’re highly crunchy). It also
forms the basis for crinoid classification. Fortunately for taxonomists
around the world, this skeleton is usually covered only with a thin epidermis
(and is thus an endoskeleton) and is clearly visible although you generally
need a dissecting microscope to see many diagnostic features. With few
exceptions, crinoid soft parts are not especially important in classification,
but this is probably because they’re a pain to deal with and few people
have bothered. We’ll discuss visceral matters later.

The crinoid body consists of three basic parts. A segmented stalk
(1) supports a cuplike calyx or aboral cup (2), which contains
or supports the viscera and from which radiate five segmented and usually
branched rays (3). Stalks consist of a series of ossicles called

columnals held together by ligaments, plus a variety of holdfast
structures. Sea lilies retain a stalk throughout their lives. Comatulids
develop a stalk following a larval stage, but shed all but the topmost
segment to take up a free-living existence as juveniles.

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