SEA-URCHINS. 



3'i 



from place to place, but always stay in one spot, where they are generally found 

 living in a hole. Sometimes the hole may have been there before the sea-urchins; 

 sometimes may have been formed by the growth of calcareous algae around the 

 sea-urchin ; but sometimes the urchin itself has bored the hole. This is accom- 



phial-shaped pourtalesia ; test with spines removed (enlarged 4 times). 



plished not by any acid secretion, — for on the west coast of Africa an Echinometra 

 has been found boring into an augite lava, — but by the continuous movement of the 

 teeth and spines. The common Strongylocentrotus is a well-known example of a 

 boring sea-urchin. When the waves wash up against the urchin it sets its spines 

 rigidly against the sides of its hole and so holds fast. 



Although most of the sea-urchins have a rigid test, yet there are some in 

 which the plates are only loosely joined together, so that the test is flexible. 



leather-urchin (§ nat. size). 



This is the case in an Astropyga; but is still more pronounced in the leather- 

 urchin (Asthenosoma), and other members of the family Ecfiinothuridce. Re- 

 spiration is effected in the regular sea-urchins by ten gills near the mouth. 

 These are thin - walled ciliated extensions of the body - cavity protruding 



