[Illustration: Fig. 114. Echinus esculentus (Lamarck), natural size.] In the prickles, or spines and tentacula (ambulacra, _feet suckers_), we see the external organs of the Echinodermata. The former are instruments of defence and progression; the latter, strange as it may appear, serve them to walk with. When it is considered that each of these prickles is put in motion by several muscles, it is impossible to repress our wonder and surprise at the prodigious number of organs brought into action in the sea-urchin. More than twelve hundred prickles have been counted upon the shell of _Echinus esculentus_, a representation of which is given in Fig. 114. If we add to this first supply of spines other smaller and in some sort accessary spines, we shall arrive at a total of three thousand prickles. Each sea urchin thus bears as many weapons as ten squadrons of lancers. When it is considered, further, that in each sucker or ambulacra there exist not less than a hundred tubes, each having an orifice, you will have a total of four thousand visible appendages upon the body of an animal of very small dimensions. If it is considered, finally, that no shell exists more admirably symmetrical, elegant, or more highly ornamental than the carapace of the urchin, it will readily be admitted that Nature has been most prodigal in her gifts to one of the humblest beings in creation--a creature which passes its existence in crawling in obscurity at the bottom of the sea. What elegance of form, eternally hidden from the eyes of man, sleeps under the heavy mass of water; and yet man imagines that everything in Nature has been created for his use and for his glory. M. Hupé records a somewhat curious observation in connection with the spines, which serve as a means of defence to the Echinodermata. He found a small mollusc, of the genus _Stelifera_, which had sought shelter in _Leixidaris imperialis_, an 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 these levelled bayonets! Numerous anecdotes are told of them. Thus: a man ignorantly put into his mouth one of these creatures, with all its prickles, and, being detected, thought himself, in his pride, compelled to swallow it because he was being looked at; immediately his mouth was full of blood. The next day he was in such a state of suffering that he could neither eat nor drink, and for a long time his life could only be preserved by nourishing injections of soup, cream, and rice.