164 THE WORLD OF THE SEA. 
are terminated by a sucker. The animals can inflate them by 
injecting into them liquid through their prickles, and by this 
means they can fix themselves to any foreign body. These organs 
are very numerous ; in the ordinary urchin there are at least 1,400, 
and in the melon echinus, about 4,300. They can move by means 
of their tentacule and their spines. Professor Edward Forbes 
once saw one crawl up the sides of a very slippery vase. 
To understand better how they use their organs of locomotion, 
imagine one at rest. All the spines are motionless, all the fila- 
ments are contracted within the shell; when the creature wishes 
to move, some of these involuntarily begin to come out; they 
extend themselves, and feel the ground all round them; then 
others follow. The animal fixes some of its tentacles to the vase 
in the direction in which it wishes to advance, these then contract, 
while the hinder ones loosen their hold, and thus the shell is drawn 
forward. The sea-urchin can thus advance with ease and even 
rapidity. During the progression, the suckers are only slightly 
aided by the spines; indeed, the latter only serve as points, upon 
which the creature rolls as if it were on stilts. It can travel as 
well on its back as on its stomach. Whatever may be its pos- 
ture, it has always a certain number of spines which are ready to 
carry it, and suckers which can fix it. In certain circumstances, 
the animal walks by turning itself round on its spines, like a wheel 
in motion. 
The mouth of the echinus is situated underneath, and is gene- 
rally at the centre. Around this orifice are fleshy tentacles, 
projecting from the surface, and more or less retractile. These 
are the organs which seize the food. 
The digestive system presents a very complicated osseous appa- 
ratus, for a long time known as “ Aristotle's lantern.” It consists 
of five pieces—the ‘eth, the plumula, the pyramids, the conpass, 
and the scythe. 
The teeth are five in number. They are all fixed on the same 
base, which is the p/wmzula, and these are situated upon the edge 
formed by the assemblage of the pyramids, which are ten in 
number, and are joined in pairs. The lower part is made firm 
by the five scythes and the five compasses. In fine, the dental 
