268 PHYLUM ECHINODERMA. 
Class HoLtorHuROIDEA. Sea-Cucumbers 
Cylindrical or worm-like Echinoderms, elongated in the 
direction of the main axis, with more or less tendency to 
bilateral symmetry, with a usually soft or leathery skin, with 
irregularly scattered microscopic calcareous bodies, with a 
terminal mouth surrounded by tentacles, with a posterior anus, 
with or without tube-feet, with no external madreporite, with 
a muscular body wall. 
The Holothurians do not at first sight suggest the other 
Echinoderms, for they are like plump worms, and the 
Fic. 138.—Spicules of Holothurians.—After Semon. 
The series 7-6 shows stages in the development of an anchor and a plate 
in a Synapta. The series A-Z# shows stages in the development of 
a wheel in CArridota, a Synaptid. 
calcareous skeleton is not prominent. But closer examina- 
tion shows the characteristic pentamerous symmetry, and 
the occurrence of calcareous plates in the skin. These 
seem to be absent in the unique pelagic Pelagothuria. 
Holothurians occur in most seas, from slight to very 
great depths. Their food consists of small animals, and of 
organic particles from the sand. Some of them catch these 
in their waving tentacles, which are then plunged into the 
pharynx. The muscles of a captured Holothurian often 
over-contract and eject the viscera at the ends or through 
