HABITS OF SEA URCHINS. 



IIT 



Class III. — BcHiif oidea {Sea-urchins). 



General Characters of Sear-TJrch.iiis. — A good idea of 



the general structure of the members of this class may 



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



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



Pig. 78.— The common Sea-urchin, BcMnvs (Strongylocentrotns) drobachiensis, 

 .<Z, frame-work of mouth and teeth seen in Tront; c, the same seen sideways; a, b, side 

 «nd external view of a Bingle 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, 

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

 hers of them assembled in large groups, feeding on fish ofEal, 

 .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 nrchfn, 

 •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 

 fcody, is not clearly understood. 



■Tooth-apparatns of the Sea- 

 , showing the complicated arrange- 

 ment of the muscles.— From Macallister. 



