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.
Fig. 115. Buccal armature of Echinus lividus.
Now let us see by what organic mechanism the urchin contrives to transport itself and walk. The tentacula, or suckers, are hollow internally, and, as we have said, are provided with small muscles. By the influx of liquid which they inclose they become inflated throughout all their prickles, 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 les Mers," thus explains the 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 filaments repose within the shell; some of these involuntarily escape; 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 filaments 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 the back or stomach; whatever their posture, they have always a certain number of prickles, 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."
Fig. 116. Masticating apparatus of Echinus lividus.
Nothing is more curious than to see a sea-urchin walk upon smooth sand. But for the colour, it might be mistaken for a chestnut with its bristling envelopes, the spines serving as feet to put the little round prickly mass in motion. They have even been observed to form themselves into a ball, and roll along like a globular fagot of prickles.
One of the most singular organs of the sea-urchin is its mouth. It is monstrous. 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. 115) projecting from the surface, the edges meeting at a point, as represented here, supported and protected by a very complicated framework, which has received the name of Aristotle's Lantern (Fig. 116). Fig. 115 represents Echinus lividus in its normal state; the other shows the masticatory organs, that is to say, Aristotle's Lantern. To give the reader a more complete idea of the buccal organ in the sea-urchin, let him glance at one from the southern seas, Clypeaster rosaceus, represented in Fig. 117, an outline of the entire animal, the buccal apparatus being placed under the shell, which has been broken in Fig. 116, so as to lay this organ bare.