224 



ZOOLOGY 



Sea 

 cucumbers 



The Cu- 



vierian 



organs 



Echinus. Others break up particles of grit, while some 

 hold small Crustacea and other animals until the tube 

 feet can reach them and pass them to the mouth. Some 

 have poison glands, and serve to repel the attacks of 

 enemies. The common California sand dollar is a very 

 flat echinoid, adapted, as are the flatfishes, for life on 

 sandy bottoms, where they offer little resistance to the 

 currents or tidal movements of the water. 



6. The Holothuroidea, shaped like a cucumber or a 

 worm, at first present no resemblance to the echinoids. 

 If we imagine an echinoid to be soft, and to be elongated 

 by pulling at the oral and aboral ends, it will assume a 

 form resembling that of a holothurian. Instead of 

 having a hard shell, the sea cucumbers possess only an 

 imperfect skeleton, usually in the form of minute spic- 

 ules, reminding us of the 

 sponges. In some cases 

 the skeletal elements are 

 entirely absent. It is dif- 

 ficult to preserve good 

 specimens of holothuri- 

 ans, because of their 

 behavior when irritated. 

 Sometimes they turn in- 

 side out, or rather extrude 

 the internal organs of the 

 body. The first parts 

 extruded are the Cuvier- 

 ian organs (part of the 

 respiratory apparatus), 

 which form a tangle of 

 sticky white thread, en- 

 veloping and rendering From Pern* 

 helpless any creature FIG. 53. A sea cucumber. 



