Page:The New International Encyclopædia 1st ed. v. 05.djvu/674

* CRINOIDEA. 582 CKINOIDEA. cilia. The feathery brandies of the arms screen food from the water, while the ciliated grooves traiis])ort the food to the mouth through the food-grooves, which are continued over the ven- tral surface from the bases of the arms to the muuth-openin';. The ventral surface of the calyx in modern genera is usually covered by a tough skin, but in the Paleozoic forms it was often covered by a superficial 'vault' or "tegmen' of calcareoiis plates. The mouth is then underneath the tegmen or ventral covering and communicates with the food-grooves of the arms through closed stony tubes. In the.se genera the plates of the vault are often so arranged as to form an ele- vated 'proboscis' (Batocrinus), at the summit of which is the anal opening, which is thus placed above the ends of the arms. The crinoid stem is attached to the base of the calyx, and consists of calcareous plates loosely joined together to allow of a considerable degree of flexibility. In- crease in the length of the stem is accomiilished by the growth of new columnar plates between the base of the ealyx and the top of the column. In form the columnar plates are discoid, and of circular or pentagonal outline, and all are pierced by a central cavity through which passes the neuro-vascular canal of the column. jNIost crinoid stems are furnisiied at their lower ends with root-like branches that serve to anchor the animal in muddy or sandy bottoms; others, Apiocrinus, have a disk-like expansion that is cemented to rocky surfaces. Several genera, especially in later geologic and modern times, have columns supplied Avith lateral branches, called 'cirri,' which ai'e similar in construction to the stem itself. The pentacrinids have the longest stems known, some of the fossil forms from the Upper I>ias rocks of Wurttemberg hav- ing been found with stems ranging from 15 feet to the extraordinary length of 50 feet. Some forms of erinoids, such as Agassizocrinus of the Carboniferous, Uintacrinus of the Cretaceous, and Antedon of recent seas, have no stem and are free-swimming animals, using their arms for locomotive purposes. Uintacrinus is the most remarkable of these, for with a body only two or three inches in diameter, it has delicate feathery tentiieles nearly six feet long, that served both as swimming-organs and as food-screens. Living Antedon, without a stem, has a whorl of cirri at the base of the calyx, and by means of these it anchors itself to the bottom. In other stemmed forms, as Woodocrinus from the Carboniferous, the base of the stem appears never to have been attached, as it ends in a simple point. The development of Crinoidea is known for only one genus, Antedon, the feather-star, and this cannot be considered as typical of the class as a whole, as it presents a case of re- gressive development. Antedon appears from the egg as an elliptical free-swimming larva that is crossed by fimr transverse ridges, has a posterior bundle of bristles and a lateral mouth, and that resembles in many respects an annelid larva. This larva increases in size, and inside of it develops an animal with the form of a cystoid, with a head of loosely Jointed perforated plates, a column, and a basal columnar plate. For a time this stalked larva is attached, and it re- sembles a primitive crinoid. but soon the stem is absorbed and the animal assumes the form of the adult free-swimming Antedon. Almost nothing is known of the ontogeny of the host of fossil erinoids. Habits akd DisTBinrTiON. The modern Coma- tulida;, like Antedon, Actinometra, etc., have a very wide distribution, and are usuall- found in waters less than 150 fathoms in depth, although one species of the Antedon has been dredged from the great depth of '2000 fathoms. The habits of Antedon are best known. It is a gregarious ani- mal and Verrill has obtained 10,000 individuals in a single trawd in the North Atlantic. It lives mostly in the upper layers of the water, but when exposed to the direct rays of the sun, it curls up its arms and sinks. When clinging by its cirri to a coral or rocky point and disturbed, it releases itself immediately and swims away by gi'aeeful movements of the arms, or crawls on its arms like a spider over the bottom. Sudden changes of temperature stun it ; it sinks to the bottom and soon dies. When these animals find themselves in uncomfortable surroundings, as when taken in the dredge and placed in aquaria, they drop off their arms, which break at specially fused joints, called 'syzygies,' but the arms are restored through regeneration if the crinoid survives. The stalked erinoids are also gregarious animals, but lliey are more restricted in their distribution, and inhabit deeper waters. The majority of fossil stalked erinoids are found in rocks that were un- doubtedly comparatively shallow-water deposits, and because the calcareous plates of the calyx, and to a lesser degree those of the stem, fall readily apart after the death of the animal, per- fect specimens are quite rare finds. The food of erinoids has been ascertained to consist of minute crustaceans, diatoms, spores of alg:e, fora- minifera, and radiolarians. Fossil Crinoids. About 175 genera and 2000 species of fossil erinoids are known. They appear first as very simple forms in Ordovician rocks, and they increase rajiidly, becoming important elements of the faunas in the Silurian, Devonian, and Carboniferous periods. In some regions they were exceedingly abundant, for their fossil re- mains form great beds of limestone known as 'crinoidal' or 'enerinal limestone,' which are found in formations of various ages. In the Silurian system alone, about 400 s])eeies. distributed among 70 genera, have been obtained from mostly three localities: the island of Gotland, the Weii- loek of England, and the Niagara gi'oup of North America. Some of the more cliaraeteristio genera are Pisocrinus, Crotalocrinus, Calceoerinus, Calli- ESPLANATION OF PLATE. 1. Anencrinite (EncrinuK), fossil in the Trias. 2. Larva of a feather-star (see Fi^s. 10 and 7). 3. Batocrintis p.rri- formis, fossil in the Subcarboniferoiis of Iowa. 4. Piso- criBU8 iJnffellifer, fossil in the Silurian of Gotland ; a, pos- terior view, of a perfect calyx ; 6. calyx seen from one side, 5. Type of calyx with a coriaceous skin in which calcareons plates are imbedded. 6. A free-swimming crinoid (Sao comya pectinata)^ fossil inthe Upper .Jurassic lithographic slates o Bavaria. 7. Diagram of arrangement of prin- cipal pieces in the calyx of a crinoid: b, basals; ih, infra- bawnls; r radials; cd, centrodorsal (compare Fig. 2 where the letters are the same, plus o, orals). 8, Woodocrinus macrodnctylus from the Carboniferous of York.shire, Eng, 9, .procrinus, A. longitudinal section through the upper- most stem-joints of Apocrinvs PSrkiBSoni (Oolitic^, show- ing empty spaces between them : B. restoration of the base of another species (ApocriDus Royssinniis. Upper .TuraV in, A feather-star (Antedon rosacea) now living on European coasts ; Fig. 2 is the larva of this, showing de- veloping plates (see Fig. 7) of the cal.vx. See article FEATHER-STAn. 11. An existing deep-sea ' stone-lily ' (ilfe- tacrinus interruptus).