THE FEEDING HABITS OF THE SEA-URCHIN. 85 



member the exoskeleton in the course of the meal, but will retain 

 such a hold of the separate parts that few, or in some cases 

 none, of them are allowed to fall to the bottom until the Sea- 

 Urchin has finished. A Sea-Urchin of 32 mm. to which one 

 of the slender antennae, 5£ inches long, of a Spinous Galathea 

 (Galathea strigosa) had been given, at once held the antenna by 

 quickly crossing those spines bordering the antenna as it lay 

 on the upper surface of the echinoid. The antenna was eaten 

 partly by this Sea-Urchin and partly by another one, which 

 rapidly approached from a distance of four inches in order to 

 join in the meal. 



Mollusca. — The remains of molluscs were found by Eichel- 

 baum in seven Sea-Urchins. They consisted of fragments of 

 lamellibranchs, and of lamellibranch and pteropod shells 

 (including Cardium and Limacina). 



All the Sea-Urchins studied by me have readily eaten the 

 soft parts of dead and gaping mussels. They have also attacked, 

 killed, and partially or wholly eaten living gastropods in large 

 numbers. For example, about sixty individuals of the gastropod 

 known as Rissoa membranacea, with an average length of shell of 

 5 mm., were placed in the aquarium containing the twelve 

 Sea-Urchins already referred to, on September 16th ; about 

 fifty specimens of Rissoa were added in October ; about fifty 

 others in December ; and several others were added on various 

 dates ; nearly two hundred in all were placed in the tank in the 

 course of four months. Many of the Rissoa were killed and 

 eaten by several Cushion-Stars (Asterina gibbosa) which lived in 

 the same tank, but about an equal number of them we're eaten 

 by the Sea-Urchins, and by the end of the following January 

 there were only about a dozen left. A Sea-Urchin has been 

 seen on the glass front of the aquarium holding as many as four 

 living Rissoce at once ; one of the molluscs, at the time the 

 observation was made, being actually held to, and partly in, the 

 mouth, whilst the others were imprisoned between the glass and 

 the spines of the Sea-Urchin. 



A Sea-Urchin will " sit " persistently upon the inverted 

 shell of a living or dead gastropod until sooner or later it is 

 able to extract the animal. A Sea-Urchin of 21 mm., which 

 was observed to be sitting on an inverted Top (Trochus zizy- 



