There is one organ peculiar to the class of Echinoderms, the general structure of which may be described here, since it is common to them all, with the exception of the Crinoids, the anatomy of which is, however, so imperfectly understood, that we are hardly justified in assuming that it does not exist even in that order. This organ is known as the madreporic body; it is a small sieve or limestone filter opening into a tube or canal; by means of this tube, which connects with the ambulacral system, the water from without, first filtered through the madreporic body and thus freed from any impurities, is conveyed to the ambulacra. In the more detailed account of the different orders we shall see what is the position of this singular organ in each group, and how it is adapted in them all to their special structure. The development of Echinoderms forms one of the most wonderful chapters in the annals of Natural History. Marvellous as is the embryonic history of the Acalephs, including all the different aspects they assume in the cycle of their growth, it is thrown into the shade by the transformations which Echinoderms undergo before assuming their adult condition. This singular mode of development, although it has features recalling the development of Jelly-fishes from Hydroids, is nevertheless entirely distinct from it, and is known only in the class of Echinoderms. As the whole story is given at length in the chapter on the embryology of the Echinoderms, we need only allude to it here in general terms. We owe the discovery of this remarkable process to Johannes Müller, one of the greatest anatomists of this century. * * * * * HOLOTHURIANS. _Synapta_. (_Synapta tenuis_ AYRES.) [Illustration: Fig. 124. Synapta, natural size.]