THE FEEDING HABITS OF THE SEA-UBCHIN. 91 



examined, the recognisable fragments may consist mainly of 

 those of animals, but observation of the habits of captive speci- 

 mens suggests that by far the greater portion of their food in 

 the sea consists of plants. The quantity of seaweed devoured 

 in the aquarium by Sea-Urchins is very large. The echinoids 

 will remain in the midst of a mass of seaweed for weeks at a 

 time, constantly devouring it, until the seaweed has become 

 reduced to heaps of faecal pellets. Any kind of seaweed seems 

 to be acceptable to the Purple-tipped Sea-Urchin. Dead or living 

 Corallina, and Zostera marina, are readily eaten. 



3. Inorganic Matter as Food. 



The Purple-tipped Sea-Urchin will swallow sand and shell- 

 gravel in the absence of other food. Eichelbaum mentions that 

 he found materials of the sea-bottom (Bodenmaterial) in eleven 

 of his Sea-Urchins, and he quotes Petersen has having found 

 fragments of calcareous matter and sand. 



It has already been mentioned that such substances as the 

 skeletons of echinoderms (p. 89), shells of molluscs (p. 86), and 

 exoskeletons of crustaceans (p. 84), consisting wholly or largely 

 of inorganic matter, are eaten by the Sea-Urchins. These 

 materials, it should again be observed, are not necessarily taken 

 only by hungry individuals, nor only by certain individuals. 

 This curious habit is being made the subject of further observa- 

 tions, but it may here be added that the Sea-Urchins will devour 

 the ordinary chalk prepared for use with the blackboard. For 

 example, on the afternoon of September 7th, a Sea-Urchin of 

 19 mm. was given a piece of chalk, measuring 39 mm. in length, 

 and 12 mm. in diameter at its thicker end. The Sea-Urchin raised 

 itself so that its teeth could be brought to bear on the rim of the 

 narrower end of the piece of chalk, and it could clearly be seen 

 to drive the points of its teeth into the chalk and then take 

 the fragments into its mouth. The animal was still upon the 

 chalk at 10 a.m. next day. The chalk was now removed for 

 examination, and it was found that it had been bitten so that the 

 portions shown in fig. 1, d had been removed. The peculiar 

 impressions made by the teeth of the Sea-Urchin are also 

 shown in the fig. (d, d). Small pieces of chalk are now given to 

 the Sea-Urchins from time to time. 



