Physical characteristics

Most sea cucumbers are soft bodied and worm- or sluglike. Some tropical species have thick, muscular body walls, whereas many deep-sea forms are gelatinous and transparent. Most species are perhaps 20 in (15 cm) long, although some apodans are as small as a few millimeters or, in one species, Synapta maculata, more than 118 in (3 m) long. Another species in the aspidochirote Stichopodidae, Thelenota anax, may weigh more than 11 lb (5 kg). Although the description "echinoderm worms" is apt, some holothuroids deviate notably from a vermiform appearance. Several burrowing forms have a foreshortened dorsum, giving them an inflated U-shaped appearance. The limit of this trend is seen in the flask-shaped members of Rhopalodinidae, the mouth and anus of which are adjacent atop a long narrow stalk. Varied development of the papillae—modified tube feet—also contributes to a diversity of form. The numerous enlarged papillae in some Stichopodidae and Synallactidae give them a prickly appearance. The elasipodans Deimatidae and El-pidiidae may have elongate ventrolaterally positioned papillae that serve as "legs," locomotor structures for raising them above, and for negotiating, soft deep-sea sediments. Most holothuroids are dark colored or, in burrowing forms, pale gray to white. In contrast, many shallow-water tropical taxa are brightly colored, being green, red, orange, or yellow. Deep-sea species often are transparent or have a violet to pinkish cast.

Close-up view of the mouth of a holothurodian (sea cucumber). (Photo by Bill Wood. Bruce Coleman, Inc. Reproduced by permission.)

The skeleton of holothuroids is considerably modified from that of most other echinoderms. In 90 percent of living species, the skeleton of the body wall is reduced to microscopic ossicles approximately 0.0004-0.04 in (10 pm-1 mm) long. The great variety in shape makes ossicles of considerable taxonomic importance. Ossicles are classified as rods, rosettes, crosses, buttons, tables, and wheels and anchors, among other shapes. In some dendrochirotes, such as the family Psolidae, ossicles may be secondarily enlarged and plate-like so that the animal is rigid. Another important skeletal feature, one definitive for Holothuroidea and used in higher-level taxonomy, is an internal calcareous ring that encircles the pharynx or throat. This ring serves as an attachment surface for muscles operating the oral tentacles and the anterior ends of other muscles that contract the body longitudinally.

As in other echinoderms, the holothurian water vascular system powering the tube feet consists of an anterior ring canal around the pharynx from which arise long canals running posteriorly. Despite their similarity to the radial canals of other echinoderms, the latter structures arise embryologi-cally in a quite different manner. For this reason these canals in holothurians have been recently renamed longitudinal canals. In holothuroids, the larval structures that would in other echinoderms extend away from the mouth and form the radial canals instead become the five primary oral tentacles. This circlet of oral tentacles, from five to more than 20 in number, is another definitive feature of Holothuroidea. The tentacles may be simple, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). In most echinoderms, the water vascular system exchanges water with the environment through a sieve plate, or madrepore, that opens externally. In most holothuroids, however, with the notable exception of elasipodans and some molpadiians, the madrepore is internal and opens into the body cavity or coelom.

Sea cucumbers, with the exception of members of Elasipodida and Apodida, have respiratory trees used in gas exchange. These structures are paired, heavily branched tubes inside the body cavity that attach to the rectum. These structures allow a type of breathing called cloacal breathing also present in an unrelated group, the echiuran worms. In many species from the mostly tropical family Holothuriidae, numerous cuvierian tubules insert at the base of the respiratory trees. These tubules apparently serve as defensive structures in most species that have them. Members of the aspidochirote Holothuriidae and Stichopodidae and the molpadiian Molpadiidae and Caudinidae have a rete mirable, a well-developed dorsal plexus of hemal vessels over the left respiratory tree, which facilitates gas exchange. Ciliated funnels, cups, or vibratile urns are small, numerous organs arranged along the insertion of the intestinal mesenteries into the body walls of the apodans Chiridotidae and Synap-tidae. Cup interiors are ciliated and appear to function in removing foreign particles from the coelomic fluid. Stato-cysts are small organs of presumed importance in balance and are arranged along the anterior radial nerves of apodan families, elasipodan Elpidiidae, and some molpadiians. Some species in the apodan family Synaptidae have "eyes" called ocelli, or optic cups. These structures are small patches of pigmented cells that enclose photosensitive cells at the base of the tentacles.

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