THE FEEDING HABITS OF THE SEA-URCHIN. 87 



subject of shell-eating by this animal, which has received little 

 or no attention, is one peculiarly deserving of study. An 

 empty shell, if it is not thick and heavy, which lies in an 

 aquarium containing a number of healthy Sea-Urchins, will 

 probably be bitten sooner or later by the echinoids. 



On August 8th an average-sized, old, and friable shell of a 



Fig. 1. — Objects eaten by Purple-tipped Sea-Urchins. a, shell of Dog- 

 Whelk ; B, last whorl of shell of Whelk ; c, right valve of shell of Mussel ; 

 D, piece of chalk. In all cases the original size of the object is indicated by 

 the thick black line, and the amount eaten is shown by the thin line, 

 lettered, a, a, a ; b, area of rugosities scraped away ; c, boring made by 

 carnivorous mollusc ; d, impressions of teeth of Sea-Urchin ; e, broken 

 part of shell not eaten by Sea-Urchins. (All natural size.) 



Whelk, to which were attached thirty-five empty sheila of the 

 Acorn-Barnacle, was put in the tank containing the twelve 

 Sea-Urchins, whose diameters ranged from 10 to 27 mm. By 

 August 20th eight of the barnacle shells had been gnawed 

 away completely, together with part of the last whorl of the 

 whelk shell (fig. 1, b). The Sea-Urchins apparently began to 

 break away the shell of the Whelk at the place where a circular 



