280 THE OCEAN WORLD. 
M. Hupé records a somewhat curious observation in connection 
with the spines which serve as a means of defence to the Echinide. 
He found a small mollusc, of the genus S#fera, which had sought 
shelter in Loxoctdaris imperialis, a sea-urchin, native of Australia; in 
a word, the interior of one of these prickles had been hollowed and 
enlarged so as to serve as a retreat for this improvised guest. 
What unexpected facts does the study of animals present! Nature 
has bestowed a protecting armour upon one little being; another 
still smaller animal discovers this, and places itself for shelter under 
the protection of one of these levelled bayonets ! 
Now let us see by what organic mechanism the sea-urchin con- 
trives to transport itself and walk. The ambulacral feet are hollow 
internally, and, as we have said, are provided with small muscles. 
By the influx of liquid which they enclose they become inflated 
throughout all their length, in such a manner that they can attach 
themselves to any solid body, at the will of the animal, by means of 
their terminal suckers. Frédol, in “Le Monde de la Mer,” thus 
explains the sea-urchin’s mode of progression. “ Let us imagine,” 
he says, “one of these creatures to be at rest ; all its spines are 
immovable, and all its feet repose within the ‘shell ; some of these 
involuntarily are protruded; they extend themselves and feel the 
ground all round them : others follow, but the animal is firmly fixed. 
If it wishes for change of place, the anterior feet contract themselves, 
whilst the hinder ones loosen their hold, and the shell is carried 
forward. The sea-urchin can thus advance with ease, and even 
rapidity. During his progression the suckers are only slightly aided 
by the spines. It can travel either on its back or stomach. What- 
ever their posture, they have always a certain number of feet, which 
carry them, and suckers, with which they attach themselves. In 
certain circumstances the animal walks by turning upon itself, like a 
wheel in motion.” 
Nothing is more curious than to see a sea-urchin walk upon 
smooth sand. But for its colour, it might’be mistaken for a chest- 
nut with its bristling envelope, the spines serving as feet to put 
the little round prickly mass in motion. They have even been 
observed to make very considerable progress under these circum- 
starices. 
One of the most singular organs of the sea-urchin is its mouth. 
It is most curious. Placed underneath the body it occupies the 
centre of a soft space invested with a thick resisting membrane : it 
opens and shuts incessantly, showing five sharp teeth (Fig. 114) 
projecting from the surface, the edges meeting at a point, as repre- 
