These are the sea-lilies or feather stars, and are thought to retain the ancestral body plan with the upwardly facing mouth. The body consists of a central disk containing the main organs, circled by 1- 200 long, feathery arms. The arms are
muco-ciliary and are branched from the five basic arms. Crinoids do not have a
madreporite, and the sexes are usually separate. In some species there is a stem anchoring the animal to the substrate. Over time there has been a trend towards free living and away from the anchored forms.
600 species survive today, but the fossil record shows many more species. They
are generally from 15 - 30 cm long, but some fossil species were 20 m long.
They are plankton feeders and are usually found below 100 m, but can also be found in shallow waters. They are particularly abundant in the tropical western Pacific.
Antedon bifia (above left) is the only crinoid found in British waters. It is fragile, so it is usually found in deeper waters away from wave action, often clinging to wrecks.
It is a free living crinoid. The 10 arms form a food-catching funnel, but are also used in locomotion. Sometimes it is found with an annelid worm attached near to its mouth. The two live commensally.
These are the
starfish or sea stars. There are about 1500 species, and range in size from 1 - 100
cm in diameter.
They have a flattened body with five (sometimes more) fairly
broad arms (see right). There is usually an ocellus (primitive eye) at the end of each arm, but their sense organs are not well-developed. The mouth is on the underside and the madreporite and anus on the
upper surface. A groove runs from the mouth to the tip of each arm, and rows of
tube feet, most with suckers, are situated along either side of the groove.
The water-vascular system opens to the outside through the madreporite. The anus is inconspicuous in most, and a few species lack both anus and intestine.
skeleton is a loosely organised series of small calcareous plates (ossicles) bound together with connective tissue, and is located beneath the epidermis which has pedicellariae (small forceps, scissors or
clamp shaped calcareous structures).
The pedicellariae keep the body surface
free of debris, and may function in prey capture; they are also used in
identification of species.
Most have separate sexes, but, some Asteroidea are hermaphrodite and are capable of releasing as many as 2.5 million eggs in 2 hours. Reproduction
is also possible by fission.
Predatory species may have an eversible stomach. This means the stomach can be inserted into any small hole in the prey's armour and digestion can proceed from there. Bivalves are commonly preyed on in this manner.
They are mainly nocturnal. Above right is Plecaster decanus found in southern Australian waters.