CYSTOIDEA. 719 CYTASE. opeiiiiifr. These furrows, representing the arms, are boniered by rows of small plates that often bear ]iiiimiles of delicale construction, as seen in Calloeystites, (ilyptosphserites, and Agela- crinus. The more primitive cystoids have neither arms nor ambulacra I furrows. The openings of the calyx are four in number. The mouth is central or subcentral on the upper or ventral surface; the anal opening is eccentric, and is generally closed by a ])yramid of small, triangu- lar plates; a third opening, often present near the anal opening and generally closed by triangu- lar jilates, is considered to be the genital orifice; and a fourth small, slit-like aperture, present in only a few genera, is of problematic nature. The calyx of the cystoid is usually elevated on a stem which often resembles that of the crinoid in being composed of a single series of plates pierced by a central canal. In some genera (as Dendrocystites) the stem is made up of plates arranged in transverse rows, and the central cavity is then much enlarged and contintious with the general cavity of the calyx. In Echino- spluvrites the stem is reduced to a tubercle on the dorsal surface, and the animal seems to have been a free living form. The discoid genera Agelacrinus and its allies are sessile, and are attached either by a pedestal or by cementation of the dorsal snriface of the calyx to foreign ob- jects, generally the shells of mollusks. Classification of the Cystoidea is a matter of difficulty, not alone because of the general im- perfection of the material, but also on account of the gi-eat diversity of structure seen within the clas's, which contains a number of synthetic or ancestral types that seem to have given rise to all the other more specialized groups of the Echinodermata. Through assimiption of a more regular arrangement of the plates and the de- velopment of the arms, with consequent rear- rangement of the ventral surface, as in Crypto- crinus. Porocrinus. and Caryocrinus. they gave rise to the Crinoidea. Reduction of the plates and enlargement of the ambulacral grooves, with the assumption of the pentameral gemmiform shape, as in Asteroblastus, leads to the Blastoi- dea. Aselacrinus is suggestive of the star-fish (Asteroidea) and brittle stars (Ophiuroidea) , and finally the Echinoidea and Holothuroidea may be imagined to have been derived from the more spherical forms of armless cystoids, the echinoids presenting a series of progressive evo- lution, the holothurians a regressive series. Range. The Cystoidea is the oldest known class of echinoderms; their isolated plates, rare- ly united to give a clue to the form of the ani- liial. known under the names of Eocystites, Pro- tocystites, etc., from the Cambrian rocks, are the earliest representatives. The class enjoyed two periods of expansion. First, in the early Ordovieian time they flourished in hosts in some regions, their remains forming the larger part of certain limestones, such as the lower Chazy limestones of Lake Chamflain. Other lime- stones, of Beekmantown and Trenton age. in the Saint Lawrence and Champlain valleys, and beds of equivalent age in the Baltic provinces of Eu- rope, contain abundant cystoid remains. The second expansion of the class occurred during the Silurian time, when these creatures lived in abundance in some portions of the seas of north- em and middle Europe and eastern ISorth America. In all about 250 species are known. and of this number only about 15 have beeir found in rocks aliove the Silurian system. The group entirely disappeared with the close of Paleozoic time. Consult : Eorbes, "On the Cystidea of the Si- lurian Rocks of the British Islands," Memoirs of the Ocolor/ical Survey of Great Britain, vol. ii.. part 2 (London, 1848); Billings, "On the Cys- tideie of the Lower Silurian Rocks of Canada," Figures and Deseriptiotm of Canadian Organic K&nains, Decade 111. (Montreal, 1858) ; Hall, "Descriptions of Some New Fossils from the Xiagara Group," Twentieth Annual licjiort of the Xcw York State Cabinet of Natural llislortf (All)any, 1807) ; Barrande, "Cystidces," Systitne SUurien du C'entre de la lioheme, vol. vii. (Prague and Paris, 1887) ; Bather, "The Cysti- dea," in Lankester's Treatifie on Zoiiiogy, part iii., chap. ix. (London, 1000) ; von Zittel and Eastman, Texlhook of Paleontology, vol. i. (Lou- don and New York, 1900). See Crinoidea; EciiixoDERjrATA ; and articles on the other classes of ccliinoderms. CYS'TOLITHS (Gk. kiJo-tis, kysiis, bladder + XWos, liihos, stone). Masses of cellulose and cukiuui carbonate (the chief constituent of lime- stones), fotmd in the cells of plants belonging to the families Urticacea; and Acanthace;c. They are most conuiion in the epidermis of both leaves and stems, but are found also in the cortex and pith. A single cj-stolith occupies a cell, nearly filling it, though the cell is enlarged. Cystoliths are irregularly warty or nodulated like a compact bunch of grapes. Each is attached to the wall of the cell in which it lies by a short stalk of cellu- lose. In reality the cystolith is an outgrowth from the cell-wall," at first peg-shaped, lat«>r club- shaped, and finally warty. Its foundation sub- stance is cellulose (the* same as that of the wall), which is impregnated with a large quan- tity of calcium carbonate in the form of very fine gi-anules, thus forming a stony mass. The stalk often contains silica. The carbonate is to lie regarded as a waste product from the chem- ical processes occurring in the plant, and is of no further direct use. It can readily be dissolved out by weak acids, the process of solution being acconipanied by effervescence and the evolution of c;irbonic-acid gas (carbon dioxide). _ Good examples of cystoliths are to be found in the leaves of nettles (Vrtica) and of the fig {Ficus). CY'TASE (from Gk. k&tos, l-ytos, cavity, cell). An enzyme that attacks the cell-walls of plants and alters the chemical composition of some of the components so that the walls swell up in water, become translucent, and finally dis- solve. The process is one of digestion. It is not yet known whether what is called cytase is a single enz-me or several, no sufficient study having been "possible. C^'tase has been found in the hyph.T of various fungi which live as para- sites "in plants, destroying their tissues. It has also been identified in "the seeds of many grasses, in which it is secreted chiefly by the 'gluten layer' (a layer of cells outside the stardi-bear- ing ones), and in the seeds of certain Legiuni- noScT, palms, etc. In many seeds the reserve food is stored as cellulose in the form of thickened walls, making the food-bearing tissue, the endo- sperm (q.v.). of bony hardness — e.g. vegetable ivory, the seed of a palm (Phytrlephaa Ivdira). In this seed certainly, and probably in all such
