CLASS HOLOTHUROIDEA. 



23 



siderably from the forms hitherto studied, they exhibit a 

 five-rayed structure in the disposition of their tentacles 

 and also of the rows of ambulacral suckers when these 

 are present. The skin, which is tough and strong, con- 

 tains numerous small calcareous plates.* The body is 

 often elongated, worm-like, and presents a marked bilat- 

 eral symmetry. 



A good idea of the Holothurian can be obtained by 

 conceiving a Sea-urchin to be drawn out from its attach- 

 ment and then laid down on its side. Five, branched, 

 retractile tentacles, which are of use in collecting small 

 plants and animals, and forcing them into the mouth, 

 take the place of the Urchin's teeth, while the hard shell 

 and long spines have disappeared. 



ORDER APODA (ap'6 da). 



Synapta (sin ap'ta) is a form found along the Atlantic 

 coast, and from its movements, 

 is ordinarily mistaken for a 

 worm. Its body is generally 

 elongated and hidden away in 

 subterranean passages in the 

 mud. It feeds by swallowing 

 sand in large quantities, leav- 

 ing the digestive organs to 

 gather from it what nutriment 

 it may contain. f 



FIG. 33. 



Syr) ap'ta gi rard'l i. GirarcTs Sea-cu- 

 cumber. A shortened individual. (Nat- 

 ural size.) A, The "anchor and plate." 

 (Greatly magnified.) 



* These often take a definite form, as 

 of " anchors," " wheels," etc. 



t Holothurians have a most singular 



habit, when alarmed, of throwing out from their bodies portions of their alimentary 

 tract. This may be a strange provision for their protection, since a preying fish 

 would seize iipon these softer morsels, leaving the tough skin, which finally re- 

 places the lost parts. Synapta, possibly for a like purpose, often divides itself 

 into numerous segments by violently mntracting. 



