HABITS OF SEA URCHINS. 



117 



CLASS III. ECHINOIDEA (Sea-urchins]. 

 General Characters of Sea-Urchins. A good idea of 

 the general structure of the members of this class may 

 be obtained by an examination of the common sea-ur- 

 chin, Echinus (Fig. 78), of the eastern coast of the United 



Fig. 78. The common Sea-urchin, Echinus (Strongylocentrotus) drobachiensis. 

 d, frame-work of mouth and teeth seen in front; <?, the same seen sideways; a, b, side 

 and external view of a single tooth (pyramid); all natural size. After Morse. 



States, Northern Europe, and the Arctic Seas. It is com- 

 mon among rocks, ranging from low- water mark to fifty or 

 more fathoms. It eats sea- weeds, and is also a scavenger, 

 feeding on dead fish, etc. We have observed great num- 

 bers of them assembled in large groups, feeding on fish offal, 

 a few fathoms below the sur- 

 face, in a harbor on the coast 

 of Labrador, where fishing- 

 vessels were anchored. 



On placing an Echinus in 

 sea-water the movements of 

 the animal, especially its 

 mode of drawing itself along 

 by its numerous long tenta- 

 cles or ambulacral feet, and 

 how it covers itself by draw- 

 ing together bits of sea- 

 weed and gravel, may be 

 observed. 



A habit less easily detected is that of some sea-urchins 

 burrowing in limestone rocks and coral reefs until the ani- 

 mal sinks quite far down. How the rock becomes thus 

 worn away, unless simply by the rotary movements of the 

 body, is not clearly understood. 



Fig. 79. Tooth -apparatus of the Sea- 

 urchin, showing the complicated arrange- 

 ment of the muscles. From Macallister. 



