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 Echinidz 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 by 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. 
