Members of this class, commonly called _sea cucumbers_, have a rather
elongate, sac-like, cucumber-shaped body and bear little resemblance to
other members of the phylum Echinodermata. The sea cucumbers do not have
a well-defined skeleton; rather the body is supported by many small,
disconnected, calcareous plates or rods called _ossicles_ or _sclerites_
(Pl. 35). These minute structures are embedded in the leathery skin
which covers the body of the sea cucumber and may be preserved as
fossils. Such remains are locally abundant in certain formations in
Texas, but because of their small size, scattered occurrence, and
problems in classification, this group is of little use to most
paleontologists.
Holothuroid body impressions have been reported from the Middle
Cambrian, and sclerites from rocks as old as Mississippian.