2 So THE OCEAN WORLD. 



M. Hupe records a somewhat curious observation in connection 

 with the spines which serve as a means of defence to the Echinidae. 

 He found a small mollusc, of the genus Stilifera, which had sought 

 shelter in Loxocidaris 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. Fredol, 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- 

 stances. 



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- 



