ECHINODERMATA 137 



name of " sea eggs," the urchins are sold during the spawning season 

 in the Orient. 



Class 4. Holothuroidea. — The sea cucumbers include rather 

 small forms which are found in the colder waters, and large tropical 

 species. Some of the Holothurians are called sea slugs because of 

 their resemblance to a mollusc. Other forms are called cotton 

 spinners because they excrete cottony filaments when irritated. 



Fig. 60. Sea cucumber. (Courtesy of Amer. Mus. of Nat. Hist.) 



The sea cucumber has a muscular body wall with a few calcareous 

 spicules, a circlet of tentacles around the mouth and five zones of 

 tube feet running from mouth to anus. {Synapta lacks tube feet.) 



The alimentary canal consists of a long coiled intestine with a 

 muscular enlargement, the cloaca, at the posterior end. Respiration 

 is carried on by the cloaca, tentacles, respiratory trees and body wall. 



General Considerations 



In the adult condition, Echinoderms usually creep along the 

 sea bottom, for they are all marine. The larvae, however, are 

 surface swimmers, or " pelagic." They are gregarious in habits 

 and found in all depths. 



Anatomy and Location. — The Echinodermata are radially sym- 

 metrical, with an exo-skeleton of calcareous plates or ossicles bearing 

 in most cases spines. They have a well-developed alimentary, 

 nervous and water vascular system, with a poorly developed vascular 

 system. Reproduction is sexual. 



Locomotion in the starfishes is a slow, creeping movement by 



