166 POPULAE HISTOPvY OF THE AQUARIUM. 



tion. Professor Eorbes saw them move by suckers. Both 

 Agassiz and Torbes had living Sea-TJrchins before them. 

 Both see the spines used ; one sees the suckers used as 

 well ; the inference is that both are used by the animal 

 for progression. Mr. Forbes says, he has seen " Echimis 

 i?iiliaris, a Spatangiis pur2mreMS, and an Aiiephidotus, all 

 walk along the bottom and up the sides of a dish of salt- 

 water by means of their inferior tentacula ; and the first- 

 mentioned anchored itself by extending and bending its 

 superior suckers, so as to reach the bottom of the dish.''^ 



The manner in which the motion by tentacles is effected 

 may be easily understood. If we observe the manner in 

 which those of a Star-fish or Sea-Cucumber elongate them- 

 selves to reach a point, and then lay hold of that point by 

 the sucker end, we shall see that when a hold is obtained 

 by a number of stretched tentacula, they have nothing to do 

 but to contract in order to draw the whole body forwards. 

 This is no doubt the way in which, a little at a time, the Sea- 

 Egg can and does move along. 



The Shell. 



The shell of the Sea-Egg is often seen ornamenting the 

 chimney-pieces of humble dwellings near the sea-side; but 



