(interambulacral plates). The arms are pierced with holes, hence their name of ambulacra; through these holes or ambulacra are protruded the sucker-like bodies just mentioned. There are ten ambulacral plates, arranged in pairs, and between these ten interambulacral plates, also in pairs; so, starting from right to left, we have two ambulacral plates united, then two interambulacral plates united, and so on around the shell. The plates are composed of still smaller pieces, these minute plates being formed through the secreting power of the skin, which dips down between the different plates. The shell of an Echinus, with its innumerable pieces, plates, spines, and suckers, is therefore quite a complex organism. If we turn to the interior of the animal, we find the intestine loosely attached, but possessing in its jaws a most complicated apparatus, the so-called Lantern of Aristotle. This lantern-shaped apparatus is composed of five triangular pieces, united at their bases. Crowning the apex of each triangle is seen a tooth, the sides of the triangle being furnished with fine saw-like teeth. The five triangles are kept firm by clamps, and movable through delicate muscles, the whole forming a most efficient, though delicate, arrangement. The nervous system is a simple ring surrounding the mouth, with radiating threads; the organs of reproduction are arranged in a direction corresponding to that of the arms. The structure of the Echinus is essentially radiate. Suppose, however, that an Echinus be drawn out until its length exceeds its breadth, and the mouth be encircled by a wreath of tentacles, we would have then a sea-cucumber, or Holothuria. (Fig. 45.) The Echinodermata agree in the structure of their water-vascular locomotor system, in the peculiar lining or hardening of the skin which incloses their bodies, and in many other respects. They may be regarded, therefore, from their structure and manner of development, as a distinct division of the animal kingdom. The origin of the Echino-
