446 THE ZOOLOGIST. 



19 mm. is excepted*), that there is a gradual decrease in this 

 proportion in passing from the smallest to the largest, the series 

 being 1'596, 0-592, 0'319, O409, 0'191, 0'084. They also show 

 that objects were much more frequently picked up and carried 

 by the smaller individuals than by the larger ones (again 

 excepting the Sea-Urchin of 19 mm.), the number of recorded 

 occasions on which the animals bore fresh materials during the 

 twenty-eight days being 17, 18, 2, 7, 7, 7, 4, 4, 4, 5, and 5. 

 Indeed, this latter difference would have been much more pro- 

 nounced if it had not been that during the first eight days a 

 source of error crept into the records ; this was that the smallest 

 Sea-Urchin, which disappeared soon after it was put into the 

 aquarium, was found later to be really present on the floor of 

 the tank, completely hidden by the pebbles which the animal 

 carried. This Sea-Urchin was afterwards usually found by 

 drawing a stick gently over the surface of the shingle, but on 

 several mornings it could not be discovered even by this means. 

 The individual of 15 mm. also was sometimes too well hidden to 

 be detected. The larger ones, on the contrary, could always be 

 seen in the first glance at the tank. Whilst the largest speci- 

 mens (27 mm.) carried an average number of three objects, the 

 smallest one (10 mm.) was almost always completely covered 

 with an average number of nine to ten objects. The two 

 smallest ones were very rarely found to be without objects, but 

 the larger ones were often to be seen in this state, and the 

 latter would sometimes allow several successive days to pass 

 without bearing any materials. 



The two small Sea-Urchins showed a very marked tendency 

 to remain on or amongst the pebbles, the individual of 10 mm. 

 being seen to climb up the face of a rock only eight times, and 

 the one of 15 mm. only four times, in the thirteen weeks. The 

 larger ones were in the habit of creeping freely all over the 

 rocks, and upon the bottom, sides, and glass front of the 

 aquarium. 



* This Sea-Urchin was extremely sluggish, often remaining in one 

 place (usually near the surface of the water) for several days in succession, 

 though it seemed quite healthy. It was only seen to carry materials 

 on five occasions in thirteen weeks. Its behaviour was probably ab- 

 normal. 



