Resembling a plant more than an animal, sea lilies are some of the most attractive but least-known animals of the deep oceans. Sea lilies are members of the class Crinoidea (phylum Echinodermata), a class that also includes the feather stars. Sea lilies are also related to more familiar echinoderms such as sea urchins, starfish, and sea cucumbers. Unlike these small, squat forms, however, the main body of a sea lily is composed of an extended, slender stalk that is usually anchored by a simple rootlike arrangement of arms. The main body, which has a jointed appearance, may reach up to 27.5 in (70 cm) in length, but most living species are much smaller. (Some fossil species have been discovered with a stalk exceeding 82 ft, or 25 m, in length.) Some sea lilies have a branched structure, while others are simple and straight in design. Sea lilies vary considerably in color, but most are delicate shades of yellow, pink, or red.
The main part of the body, the calyx, is carried at the top of the stalk, rather like a crown. This contains the main body organs and is further developed with a series of 5-10 featherlike arms. The number of arms appears to vary with water temperature: some of the larger, tropical species may have up to 200 arms. Each arm is further adorned with a large number of delicate pinnules which, when extended, increase the area available for trapping food. When the animal is not feeding, or if the arms are in danger of being eaten by a predatory fish or crustacean, the arms may be folded and the entire crown withdrawn. The mouth is located in the central disk at the base of these arms. The arms and pinnules together trap fine particles of food from the swirling water currents. Tiny grooves on the surface of each pinnule lead into larger grooves on the main arm, like streams joining a river, and continue across the surface of the calyx to the mouth.
Rather than being composed of living tissue, much of the body is made up of calcium carbonate, which provides a rigid framework that supports the head of the animal. Within this protective armour, the actual movements of the sea lily are restricted to simple bending, unlike the movements of feather stars, which are mobile and may move from safe resting places to an exposed site for feeding purposes.
Until recently, most sea lilies were only known from fossil remains. These species appear to have been quite abundant at certain times in the geological history of Earth. Today, some 80 species are known to exist. Despite this, little is known about these animals, largely because the vast majority tend to live in deep ocean trenches, often at depths of 3,935-4,265 ft (1,200-1,300 m) and occasionally as deep as 29,530 ft (9,000 m). Virtually no light penetrates the water at these depths, and living organisms are few and widely scattered. Most species living at such depths need to conserve their energy, and sea lilies, by virtue of their few living organs and tissues, probably have a very low rate of metabolism. Most of the food they receive comes in the form of "fecal rain" from the upper water levels: as animals and plants die, parts of their bodies fall through the water column where it is scavenged by other organisms. Although scavenging animals are widespread and numerous in the oceans, some of these materials do eventually reach the deepest regions and, in so doing, ensure a steady if limited supply of foodstuffs to specialized species such as sea lilies.
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