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." 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. [Illustration: Fig. 115. Buccal armature of Echinus lividus.] 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. [Illustration: Fig. 116. Masticating apparatus of Echinus lividus.] The shape of the _Clypeaster rosaceus_ is oval, straighter in front, and thick and rounded at the edges. It is more common and more largely distributed than any other living species, and it is supplied with four or six ambulacra, or feet.