Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/660

638 638 ECHINODERMATA liert s beads.&quot; Among the various forms by which the Crinoidea were represented during the Mesozoic period, towards the close of which they diminished materially in number, are Encrinus (Muschelkalk) ; Extracrinus, Pcntacrinus (Lias) ; ComatuHdcc (Rhaetic) ; Apiocrinus, Millericrinus, Eugeniacrinus, and such forms as Saccosoma and Ptcrocoma, related to Comatula (Jurassic) ; and Bourgueticrinus, Marsupitcs, and (?) Comatulidce (Chalk). The Tertiary genera are but few. They include Bourgueticrinus and Cuinocrinus (London Clay), and Comatula (Coralline Crag.). Pen- tacrinus, Rhizocrinus (allied to Apiocrinus and Bourgueticrinus) and the related forms Bathycrinus and Hijocrimis, the sessile genus Jfolopus, Actinomctra, and Comatula are living forms. Order V. CYSTIDEA. The body in the Cystidea is in most cases rounded, and is covered with polygonal plates ; it is attached by a stem, and may be provided with arms or pinnules developed from the upper or oral surface. The stem is short, usually similar to that of the Crinoidea in construction, but with out cirri ; the joints are rounded, and sometimes moniliform, and usually become , broader but thinner towards the base of the body. In Ateleocystites (see H. Woodward, Geol. Mag. 1871, p. 71) the calyx is com pressed laterally, and shows sculpturing similar to that of the plates of the peduncu- lated Cirripedia. The plates of the calyx are pentagonal, hexagonal, or imperfectly triangular, and are closely united together ; they vary in number, and in Sphceronites (1 in fig. 28) are very numerous. In Cryp- tocrinus the calyx is composed of three rows of plates, which may be regarded as basalia, parabasalia, and radialia. According to Professor K r orbes (Mem. of the Geolog. 2. Psemiocrinus M- Survey of Great Britain, 1848, vol. ii. part gSSSJJSE 2), the following series of plates may be lc y- generally distinguished : a basal series; subovarian, centro- latcral, and supra-ovarian series on a plane below, on the same plane with, and on a plane above the ovarian pyramid respectively ;circa-ovarian plates or ossicles, encir cling that structure; and oral plates, immediately surrounding the mouth, which vary considerably i i uuraber(fig.29). The plates are frequently ornamented with grooves and tubercles. Arms and pinnules are not uni versal. In Comarocystites punctatus, Billings, the arms are free; sometimes they are wanting, and the FIG, 28. Cystidea. antium, Wahl. ; L. Silurian, Sweden. FIG. 29. Echinoencrinus armatus ; dissection and projection of plates. (After Forbes.) pinnules are attached to 1 - 4 - basal; 5-9 subnvarian; 10-14, centro- .., lateral; 15-10. supra-ovarian; r, genitil the Upper portion OI the pyramid and p ates. Plates ], 5, 14, and calyx. Commonly tho bear semi-rhombs. arms resemble ambulacra, and are reflected towards the base of the calyx, and closely applied to its surface. Pores on the antambulacral surface may be absent (Cryptocrinus), irregularly scattered (Caryocrinus), in pairs (Sphceronites), or, as in Pseudocrinus (2 in fig. 28), Echinoencrinus, and other genera, slit-like, and arranged to form &quot; pectinated rhombs,&quot; or &quot; hydrospires,&quot; the two halves of each rhomb being on separate plates. In Caryocrinus ornatus there are thirty pectinated rhombs, con sisting each of a number of parallel internal flat tubes communicat ing at _both ends with pores opening internally. The rhombs in Fleurocystitcs are not tubular as in Caryocrinus, but are made up of numerous parallel inward folds of an exceedingly thin part of the test. Again in Paltcocystites tenuiradiatus the whole surface heirs rhombs, which, when uninjured, have a complete though very thin calcareous covering, and communicate by a small pore at their base with the body-cavity. (See Billings, A nn. and Mag. of 2^ at Hint. 1870, p. 259-61.) What is usually regarded as the mouth is situated in the centre of the ventral surface of the calyx, opposite the point of attachment of the stem, and from it radiate the furrows for the arms, when those appendages are present. It is apparently small and circular in Sphceronites; in Caryocystitcs it is transversely elongated and lobed; in Hemicosmites elevated on a proboscis ; in Echinoencrinus usually longitudinal and bordered by peculiar plates. A small perforation alongside the mouth, considered to be the anus, is generally present. It has sometimes, as in Echinoencrinus (fig. 30) and Apiocystites, the form of &quot;an FIG. 30. Echinoencrinus armatus. (After Forbes.) a, mouth ; b, anal aperture ; and c, ovarian pyramid of the same, enlarged. arched or crescentic groove terminating apparently at each end with a pore, and having united with it, or placed a little below it, an orifice in the middle line of a suture, as if in the junction of tvo oral plates &quot; (Forbes). Almost invari ably, on the oral portion of the body, interradially placed, is a round or oval aperture covered by a pyramid of 5 or triangular valves. This in the opinion of most authorities is probably the ovarial orifice ; but according to Mr Billings it is an oro-anal aperture, the central opening or mouih above mentioned being an &quot; ambulacral orifice.&quot; Cystid&amp;lt;a first occur in strata of Cambrian age. They especially characterize the Lower and Upper Silurian series of rocks, and apparently died out in the Carboniferous epoch. Order VI. EDRIOASTERIDA. Under this head are grouped such forms as Edrioaster, Agelacrinites, and Hemicystitcs. The shape is that of a rounded star-fish or flattened echinus with a concave base. There is an ovarian pyramid, but stem and arms are wanting, and tho ambulacra communicate by perforations with the calycii.c cavity. The Edrioasterida are exclusively Palreozoie. Their nearest living ally is the Australian species I/yjjoiwme Sarsii, Lov., which approaches Agelacrinites m form. Order VII. BLASTOIDEA. In this group of fossil Echinoderms the bud-shaped or prismatic, armless, and closely plated calyx is supported on a short, jointed stem. Of the three basal plates in Pentremites two are double ; succeeding the basals is a row of five pieces, and into the deep clefts of the upper portion of these fit the lower ends or apices of the ambulacra ; a FlG - third series of five small, deltoid, inter- radial plates occupy the spaces between the oral portions of the ambulacra. The ambulacra, or &quot;pseudambulacral areas,&quot; present a superior surface formed by a double series of ossicles running from a median line to the border, where they support pinnules; beneath the ossicles is usually a lanceolate plate formed in many if not all species of Pentre mites, of two contiguous plates, and edged by a simple row of transverse pieces, which are pierced with marginal pores. Each row of pores opens below into one or more flat canals, or, according to the definition of Billings (Ann. and Mag. of Nat. Hist., vol. v. 4th ser. p. 263), into a &quot; hydrospire &quot; consisting of &quot; an elongated internal sac, one side of which is attached to the inside of the shell [or Pentre - mite