The most important feature distinguishing the sea cucumbers is a calcareous ring that encircles the pharynx or throat. This ring serves as an attachment point for muscles operating the oral tentacles and for the anterior ends of other muscles that contract the body longitudinally. Sea cucumbers are also distinct as echinoderms in having a circlet of oral tentacles. These may be simple, digitate (with finger-like projections), pinnate (feather-like), or peltate (flattened and shield-like). A third key feature, found in 90% of living species, is the reduction of the skeleton to microscopic ossicles (Figure 1). In some species, the ossicles may be enlarged and plate-like.
Figure 1. Calcareous skeletal ossicles from the body wall (in situ) of two recent holothurians.
Left: Wheels and hook-shaped rods of Trochodota allani (Apodida: Chiridotidae).
Right: Spinose wheels with perforated hub and simple rods of Siniotrochus phoxus (Apodida: Myriotrochidae).
Photographs copyright © 2000 Mike Reich.
As in other echinoderms, the holothurian water vascular system consists of an anterior ring canal from which arise long canals running posteriorly (not shown in Figure 2). Despite their similarity to the radial canals of other echinoderms, these latter structures arise embryologically in a quite different manner. For this reason these canals in holothurians have been recently renamed longitudinal canals (Mooi and David 1997). In holothurians, the larval structures that would form the radial canals in other echinoderms instead become the five primary tentacles. Also, holothurians with the exception of members in Elasipodida have a madrepore that opens into the coelom (body cavity). In contrast, elasipodans and nearly all other echinoderms have a madrepore that opens externally.
Figure 2: Main internal anatomical features of a cucumariid sea cucumber (Dendrochirotida).
Drawing by Ivy Livingstone. Copyright © 1995 BIODIDAC.
Some sea cucumbers possess organs not found in other invertebrates. In some Aspidochirotida, the respiratory trees display Cuvierian tubules. In most species, these are apparently defensive structures. They can be expelled through the anus, whereupon they dramatically expand in length and become sticky, entangling or deterring would-be predators, such as crabs and gastropods. Many forms, with the exception of members of Elasipodida and Apodida, possess respiratory trees used in gas exchange. These are paired, heavily branched tubes attached to the intestine near the anus. This type of breathing ("cloacal breathing") is also present in an unrelated group, the echiuran worms.
Hyman (1955) provides a useful account of holothuroid gross anatomy, Smiley (1994) covers microscopic aspects, while Smiley et al. (1991) reviews reproduction and larval development.
No one has provided updates yet.