Then again, owing to the possession of ossicles, the Echinodermata are one of the few groups of Invertebrata of which abundant remains occur fossilised. In attempting, therefore, to decipher the past history of life from the fossil record, it is necessary to have an exact and detailed knowledge of Echinoderm skeletons and their relation to the soft parts. Lastly, the internal organisation of Echinoderms throws valuable light on the origin of the complicated systems of organs found in the higher animals.
Echinodermata are divided into two great sub-phyla, which must have very early diverged from one another. These are:—
(1) Eleutherozoa,
(2) Pelmatozoa.[[439]]
The sub-phylum Pelmatozoa, to which the living Feather-stars (Crinoidea) and the majority of the known fossil species belong, is characterised by the possession of a fixing organ placed in the centre of the surface opposite the mouth—the aboral surface as it is called. Ordinarily this organ takes on the form of a jointed stalk, but in most modern species it is a little knob with a tuft of rooting processes, termed cirri. In the other sub-phylum, the Eleutherozoa, no such organ is found, and the animals wander about freely during their adult life, though for a brief period of their larval existence they may be fixed by a stalk-like protuberance arising from the oral surface.
SUB-PHYLUM I. ELEUTHEROZOA
The Eleutherozoa are divided into four main classes, between which no intermediate forms are found amongst the living species, though intermediate types have been found fossil.
The four classes into which the Eleutherozoa are divided are defined as follows:—
(1) Asteroidea (Starfish).—"Star"-shaped or pentagonal Eleutherozoa with five or more triangular arms, not sharply marked off from the central disc. The mouth is in the centre of one surface, called from this circumstance the "oral"; the anus is in the centre of the opposite surface, termed the "aboral." From the mouth a groove runs out on the under surface of each arm towards its tip, termed the "ambulacral" groove. Projecting from the ambulacral groove are found the podia or tube-feet, the organs of movement and sensation of the animal.
(2) Ophiuroidea (Brittle Stars).—Eleutherozoa, in which the body consists of a round disc with long worm-like arms inserted in grooves on its under surface. No anus is present, and the ambulacral grooves are represented by closed canals. The podia are merely sensory and respiratory, locomotion being effected by muscular jerks of the arms.