I never could understand why the dental framework of the sea-urchin has been called Aristotle's Lantern, for this formidable apparatus resembles the front view of a battery of cannon more than a lantern. It consists of a series of pieces designated by the names of compass, scythe, pyramid, and plumula, which it would serve no useful purpose to describe. [Illustration: Fig. 117. Clypeaster rosaceus (Lamarck).] We have said that the mouth of the urchin is monstrous in proportion to its size, and the teeth of proportionate dimensions. As these project from a very formidable mouth, one can easily be assured of the sharpness of their extremities by intruding his fingers on them. In fact, it is necessary that these organs should be singularly powerful, because, as we shall see farther on, the sea-urchin makes incisions in the solid rock with them, and hollows out shelter for himself. The strong and sharp teeth grow at the base in proportion as they are used at the points, as is the case with some of the rodent mammalia. By this means they are always sharp and in good condition. Five groups of powerful muscles are used to work these terrible grinders. To this formidable mouth is attached an œsophagus or gullet, and an intestine which extends along the interior walls of the carapace, describing the circumference of its principal contour. The regimen of the Echinidæ is still imperfectly known; nevertheless, from the presence of shells, fragments of corals, crustaceans, and even other Echinodermata in their intestinal tube, it is to be inferred that a certain number of them at least are carnassiers, or flesh-eaters, while others are supposed on the same evidence to be vegetarians. The organs of respiration of the Echinidæ appear to be certain flattened vesicles in the form of very delicate laminæ, which adhere to the internal surface of the walls of the body, and float freely in the liquid with which the visceral cavity is filled. These organs, known as the internal _branchiæ_, are in communication with the central canal and ambulacral tubes. The heart is spindle-shaped, tapering above, swelling below. There are two distinct vascular systems, one intestinal, the other cutaneous. [Illustration: Fig. 118. Skeleton and Masticating Apparatus.]