Invertebrates

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Feather Stars

Feather stars


Kingdom: Animalia

Phylum: Porifera

Class: Crinoidea


Common name(s):  Feather stars, Crinoids or Sea lillies


General Details


Crinoids are marine animals that live both in shallow water and in depths as great as 6,000 meters (20,000 ft). Sea lilies refer to the crinoids which, in their adult form, are attached to the sea bottom by a stalk. Feather stars refer to the unstalked forms. 


Crinoids are characterized 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 recognized, 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, 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.


Descriptive Details


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 1 meter (3.3 ft) 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. 


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.



Data provided by:

Wikipedia / Accessed December 2014

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