THE SEA-URCHINS. 



337 



sally recognised as organs subservient to the nutrition of the 

 animal, and destined to seize the food floating by, and to convey 

 it to the mouth, one passing it to the other. Even in their out- 

 ward appearance, the sea-urchins are not so very different from 

 the sea-stars as would be imagined on seeing a Butt-thorn near 

 a globular urchin, for both orders approach each other by 



Shell of Echinus, or Sea-Urchin. 

 , On the right side covered with spines, on the left the spines removed. 



gradations; thus, the Goniasters, with their cushion-shaped 

 disks and shortened rays, approximate very much in shape to 

 the sea-urchins ; and among the latter we also find a gradual 

 progression from the flattened to the globular form. Still 

 there are notable differences between the two classes. Thus in 

 the sea-urchins the digestive organs form a tube with two 

 openings, while in the true sea-stars they have but one single 

 orifice. Their mode of life is, however, identical. 



The Echinidse move forward by means of the joint action of 

 their suckers and spines, using the former in the manner of the 

 true star-fishes, and the latter as the snake-stars. They also 

 make use of the spines, which move in sockets, to bury them- 

 selves in the fine sand, where they find security against many 

 enemies. 



Some species even entomb themselves pholas-like in stone, 

 inhabiting cavities or depressions in rocks, corresponding to 

 their size, and evidently formed bv themselves. Bennett de- 

 scribes each cavity of the edible Echinus lividus as circular, 

 agreeing in form with the urchin within it, and so deep as to 

 embrace more than two-thirds of the bulk of the inhabitant 



