98 TEE ZOOLOGIST. 



are at right angles to the floor, and then eat along the seaweed. 

 A Sea-Urchin will often lie on its aboral surface whilst eating 

 inconveniently placed material ; and in order to hold and 

 eat a bulky piece of food will sometimes climb a vertical rock 

 with its mouth directed outwards. A living Ascidia of 30 mm. 

 in length was placed upon the uppermost part of a Sea-Urchin 

 of 23 mm., which was climbing up a vertical rock, and the Sea- 

 Urchin, which seemed quite able with its spines to hold the 

 smooth Ascidian, at first endeavoured to place its prey beneath 

 the mouth, but the Ascidian was too bulky to lie between the 

 Sea-Urchin and the rock. The Sea-Urchin then slowly turned 

 completely over, holding firmly to the rock and the prey, and the 

 echinoid began to eat the Ascidian as soon as the former lay with 

 its mouth outwards. The whole process, from the time of placing 

 the Ascidian to that at which the Sea-Urchin began to eat, 

 occupied between nine and ten minutes. 



If the food can be moved the Sea-Urchin will often arrange 

 it conveniently under its teeth. A Rissoa, for example, is 

 often, but not always, turned in such a way that the pointed 

 end of the shell is directed towards the mouth, and the 

 first (or first and second) whorl of the shell then bitten off. 

 A few Risso<z which had escaped after being so treated have 

 been seen in the aquarium. As was pointed out on p. 88, 

 the hinder parts of the shells of Mussels have been bitten by 

 the Sea-Urchins, and it is probable that this part of the 

 shell was selected because the echinoids were better able to 

 seize with their teeth the narrow edges of the shells than they 

 would be to seize a thicker part. The inverted gastro- 

 pods on which the Sea-Urchins so often " sit " are probably 

 turned over by the echinoids, though the latter have not 

 actually been seen to do this. A straggling exoskeleton of a 

 shrimp will be gathered into a compact mass beneath the 

 mouth. A Sea-Urchin which found the tube of a Sabella 

 penicillus (whose length was four times the diameter of the 

 echinoid) inconveniently long, twisted the tube around its body. 

 A Sea-Urchin of 10 mm. (carrying eleven pebbles of nearly one 

 and a half times its own weight) remained for several hours 

 balanced on a large horizontally projecting Sabella, in whose 

 tube a hole was bitten. 



