THE PHYLA HEMICHORDATA AND ECHINODERMATA 373 



Holothurians are remarkable for the ease with which they will 

 throw away their viscera. Whenever conditions are unfavorable, whether 

 this be clue to lack of oxygen, high temperatures or excessive irritation, 

 the sea cucumbers contracts violently and ejects the entire digestive 

 tract. In different species it may be thrown out through the mouth, 

 through the anus, or rupture through the side of the body. Later a new 

 digestive tract is regenerated. Spontaneous evisceration has been sug- 

 gested to be a device by which the sea cucumbers offers a morsel to a 

 potential predator on the chance that the less succulent body wall will 

 be left unharmed. When evisceration is found in nature, however, it is 

 usually associated with unfavorable environmental conditions. Possibly 



Figure 19.12. Larvae of the sea cucumber, Cucumaria frotidosa. Left, "sitting," and 

 right, "walking." (After Runnstroin and Runnstrom.) 



by throwing out the viscera sea cucumbers can close up tightly and live 

 at a reduced metabolic level until favorable conditions return. 



When larval sea cucumbers settle to the bottom, the first tube feet 

 to develop are five around the mouth and a single pair near the anus 

 on the lower side. These tiny holothurians clamber about actively, often 

 walking on the pair of posterior tube feet (Fig. 19.12). This is the only 

 example of bipedal locomotion known in the invertebrates. 



170. Class Echinoidea, the Sea Urchins, Heart Urchins and 

 Sand Dollars 



The body skeleton of the echinoids forms a rigid box (Fig. 19.5 C). 

 Five ambulacral grooves with tube feet radiate from the mouth up 

 around the sides to end near the anus. The tube feet on the lower 

 surface usually have suckers and are used in locomotion whereas the 

 lateral and upper tube feet are often long and filamentous, apparently 

 used for respiration. 



Sea urchins have numerous long spines, some of which aid the tube 

 feet in walking. The urchins creep slowly about, using their five sharp 

 teeth to scrape and chew whatever they pass over. The skeleton and 

 musculature associated with the teeth form a distinctive structure known 

 as Aristotle's lantern (Fig. 19.13). 



