The Radiata, or radiated animals, so designated because the parts of which the body is composed are arranged around a common centre or axis, are divided into three sub-classes; namely, 1, the _Polypifera_, whose fossil remains were treated of in the previous chapter; 2. the _Acalepha_, or Jelly-fishes, whose structures are so perishable as to render it improbable that any vestiges of them will be found in a fossil state, though imprints of the general outline of certain kinds may possibly occur;[264] and, 3, the _Echinodermata_, comprehending under that term the _Stelleridæ_, or Crinoids and Star-fishes, and the _Echinoderms_, properly so called. This sub-class is the subject of the present section; it comprises four orders, viz. Crinoidea, or Lily-like Animals. Asteriadæ, or Star-fishes. Echinidæ, or Sea-urchins. Holothuriadæ, or Sea-slugs. [264] The impression of an Acaleph resembling an _Æquorea_ (a kind of Medusa), is stated by M. Pictet to have been observed in a slab of schistose rock, in Germany. Vestiges of the Stellerida are among the earliest relics of animal organization hitherto discovered. Many kinds of Crinoidea abound in the Silurian rocks, and one genus of Star-fishes occurs in the same deposits: the Echinidæ first appear in the Devonian formation. The Echinodermata possess the radiated type of structure in an eminent degree; especially the Asterioidæ and the Echinidæ, of which the common Star-fish, and Sea-urchin, are familiar examples. The external integument or skin is in many kinds protected by spines, (hence the name Echinoderms or spiny-skin,) and perforated by numerous foramina for the imbibition and transmission of sea-water, and for the protection of minute soft tubular processes (called _pseudopodia_), which constitute organs of adhesion and locomotion. The first two orders have endo-skeletons, composed of numerous ossicula or little calcareous bones: in the third order, the Echinidæ, the body is inclosed in a calcareous case or shell, formed of numerous plates closely adjusted to each other; in the fourth order, the Sea-slugs, the body has only a tough outer integument without movable spines. Diversified in form and external appearance as are the Invertebrata thus grouped together, they are naturally related by their organization. The Crinoidea may be regarded as Star-fishes fixed to one spot by a jointed stem; the Star-fishes as free Crinoidea; the Echinidæ as Star-fishes with the rays coalesced and united into a globular or spherical case; and the Holothuriæ, as elongated Sea-urchins, destitute of spines, and without a calcareous envelop. [Sidenote: CRINOIDEA.]