|
Crinoids are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). Crinoidea comes from the Greek word ''krinon'', "a lily", and ''eidos'', "form".〔''Webster's New Universal Unabridged Dictionary.'' 2nd ed. 1979.〕〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=crinoid&allowed_in_frame=0 )〕 They live both in shallow water and in depths as great as . Sea lilies refer to the crinoids which, in their adult form, are attached to the sea bottom by a stalk.〔(【引用サイトリンク】url=http://www.britannica.com/EBchecked/topic/530621/sea-lily )〕 Feather stars〔(【引用サイトリンク】url=http://www.britannica.com/EBchecked/topic/203206/feather-star )〕 or comatulids refer to the unstalked forms. Crinoids are characterised by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognised, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults. There are only about 600 extant crinoid species,〔(【引用サイトリンク】title=Animal Diversity Web - Crinoidea )〕 but they were much more abundant and diverse in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are almost entirely made up of disarticulated crinoid fragments. ==Morphology== Crinoids comprise three basic sections; the stem, the calyx, and the arms. The stem is composed of highly porous ossicles which are connected by ligamentary tissue. The calyx contains the crinoid's digestive and reproductive organs, and the mouth is located at the top of the dorsal cup, while the anus is located peripheral to it. The arms display pentamerism or pentaradial symmetry and comprise smaller ossicles than the stem and are equipped with cilia which facilitate feeding by moving the organic media down the arm and into the mouth. The majority of living crinoids are free-swimming and have only a vestigial stalk. In those deep-sea species that still retain a stalk, it may reach up to in length, although it is usually much smaller. The stalk grows out of the ''aboral'' surface, which forms the upper side of the animal in starfish and sea urchins, so that crinoids are effectively upside-down by comparison with most other echinoderms. The base of the stalk consists of a disc-like sucker, which, in some species, has root-like structures that further increase its grip on the underlying surface. The stalk is often lined by small cirri. Like other echinoderms, crinoids have pentaradial symmetry. The aboral surface of the body is studded with plates of calcium carbonate, forming an endoskeleton similar to that in starfish and sea urchins. These make the calyx somewhat cup-shaped, and there are few, if any, ossicles in the oral (upper) surface. The upper surface, or ''tegmen'', is divided into five ''ambulacral areas'', including a deep groove from which the tube feet project, and five ''interambulacral areas'' between them. The anus, unusually for echinoderms, is found on the same surface as the mouth, at the edge of the tegmen.〔 The ambulacral grooves extend onto the arms, which thus have tube feet along their inner surfaces. Primitively, crinoids had only five arms, but in most living species these are divided into two, giving ten arms in total. In most living species, especially the free-swimming feather stars, the arms branch several times, producing up to two hundred branches in total. The arms are jointed, and lined by smaller feather-like appendages, or ''pinnules'', which also include tube feet.〔 Image:Crinoide vraie.jpg|Stem, theca and arms of a "true" (stalked) crinoid (family Isselicrinidae). Image:Comasteridae - Oxycomanthus bennetti-001.jpg|''Oxycomanthus bennetti'' (comatulid) Image:Lamprometra palmata hgsus03.JPG|Tegmen of a ''Lamprometra palmata''. The mouth is located at the center of the 5 feeding grooves, and the anus at the top of the column. Image:Elegant feather star9.jpg|Close-up on the cirrhi that allow comatulids to walk and attach themselves. Image:Myzostoma fuscomaculatum at Percys Hole detail.jpg|Close-up on the pinnules of a ''Tropiometra carinata'' (with parasites ''Myzostoma fuscomaculatum''). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「crinoid」の詳細全文を読む スポンサード リンク
|