THE FEEDING HABITS OF THE SEA-URCHIN. 98 



surmount numerous obstacles, such as piles of rocks, which may 

 lie in its path. Descriptions of two experiments upon the ability 

 of captive Sea-Urchins to detect and eventually to reach distant 

 food will here be presented. 



First experiment. — A plan of the glass front of the aquarium is 

 given in fig. 2. The tank contained about 2G gallons of thoroughly 

 aerated sea-water, and the water was kept in constant movement 

 by means of an air-pump. Of the twelve Sea-Urchins (which 

 had been in captivity for twenty-nine weeks, and were perfectly 

 healthy), four were visible, and the other eight were hidden 

 behind the piled rocks at the back of the aquarium. Two of the 

 visible Sea-Urchins, each of 23 mm. in diameter, were on the 



I 



I 





;>'3^ 



"\Z 



/ 









1 



1 



V 



\ 



\ 

 \ 





B-' 





b 



C." 



- A«^^'4;i 



ii. 



31" 



Fig. 2. — Plan of glass front of aquarium to show tlie routes taken by 

 four Sea-Urchins when seeking Laminaria. Thick black line, frame of 

 aquarium ; thin line, surface of water ; dotted lines, routes of Sea-Urchins ; 

 A, B, C, D, starting points of Sea-Urchins ; 1, position of Laminaria. 



glass at the places indicated by letters A and B ; a Sea-Urchin 

 of 16 mm. was partly imbedded in the pebbles of the bottom at 

 C, immediately behind the glass ; and one of 19 mm. upon the 

 pebbles, also just behind the glass, at D. The time selected.for the 

 beginning of the experiment was when the Sea-Urchins were 

 found in a quiescent state, in order that the first signs that the 

 animals had discovered the presence of the food might be 

 observed. Three large fronds of Laminaria were placed at 

 9.30 a.m. on the floor of the tank immediately behind the glass 

 at the place numbered 1. The routes followed by the Sea- 

 Urchins in travelling to the food are indicated by dotted lines. 



