SEA-URCHINS. 3 n 



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 Eckinothuridce. 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 



