Cystoidea

Cystoidea, a class of Echinodermata of great interest to the student, as it appears to include the nearest form known to the ancestral Echinoderm. The whole class is extinct, and was confined to the Palaeozoic era, being most abundant in the Silurian. It is hardly possible at present to draw up any precise definition of the Cystoidea, and in all probability the group of forms now included under this name will have to be broken up. The Cystoids are usually spherical or ovate in form, and are composed of a variable number of plates irregularly arranged and with the radial symmetry typical of the Echinoderms imperfectly developed. The mouth opens on the centre of the upper surface; around it there may be a circle of arms; the arms are often bent back and soldered to the plates of the body (as in Callocystis) or they may be absent; in the former case the arms play the part of food grooves. On the side of the body there are other openings, as to the true functions of which opinions differ; one of them is probably anal and another ovarian. In many Cystoids there is a long stem (e.g. in Caryocrinus) and this may end in a spindle-shaped solid piece (e.g. in Lepadocrinus); in others the stem is absent. Most of the plates are perforated by series of pores; the function of the pores probably varied in different groups, but in most they appear to have been connected to a series of respiratory tubes known as the "hydrospires." In the most typical Cystoids - the "Rhombifera"' - the pores are fairly large, and each opens in a rhomb-shaped area or "pore rhomb"; such e.y. is the case in Glyptocystis, and these almost certainly opened to hydrospires: in others the pores are small and numerous, as in Callocystis. In another group - the Diploporitidae - the pores are in pairs, as in Glyptosphaerites, and these may represent tube feet. In the third order or Aporitidae the pores are absent. The affinities of the Cystoids are at present very uncertain; in some, as Atelecystis, the arrangement of the plates is so irregular that the radial arrangement of the internal organs that is so characteristic of the Echinoderms had probably not been developed. Most of the higher Cystoids are clearly related to the stalked Echinoderms or "Pelmatozoa" (Sea Lilies, etc.); hence the whole group was once included in this sub-class. But some Cystoids such as Lichenoides had no stem, and consequently cannot be included in the Pelmatozoa. The Cystoids are represented by some imperfectly known genera in the Cambrian, attained their maximum in the Ordovician and Silurian, and died out in the Carboniferous, the last genus being the remarkable form known as Agelacrinus. A few years ago the discovery of a living Cystoid was announced: it was named Hyponome, but it was subsequently proved to be only the central visceral mass of a "Rosy Feather Star" (Antedon) separated from its arms and skeleton.