Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/655

633 E CHINOD-ERMATA 633 FIG. 12. Section of ray of Astropecten aurantiacus. a, vertebral ossicles; b, adambulacral ossicles: c, d, marginal ossicles; e, paxillEe. (After Gaudry.) internal ciliated epithelium. Studding the perisome are numerous spines, attached to the ossicula on the dorsal surface and to those bordering the ambulacral grooves ; sometimes also there are tufts of bristles, the paxillce. The pedicellariie are attached to the perisome and spines, and are either sessile or provided with short foot-stalks. Ex cept iu one group, they have two blades only, which are moved by divaricator and adductor muscles. The lower or oral surface of the star-fish with the ambu lacra corresponds to the ambulacral, the aboral or ant- ambulacral surface to the interambulacral areas of the echinus. The deep ambulacral grooves which occupy the middle of the lower face of each ray are formed each by a series of plates, the vertebral ossicles, articulated to one another by their inner opposed ends, and united by their louver or outer ends to rows of plates, the adambulacral ossicles, which form the margins of the grooves, and are themselves succeeded by one or more series of marginal ossicles (fig. 12). The outer ends of the innermost pairs of ambulacral ossicles unite round the mouth to form five crests, which bear spines and peclicellame. On the aboral sur face of the body are the tergal plates (fig. 13). Transverse muscular fibres unite the lateral halves of the arm-segments ; similar fibres supply the floor of the ambulacral groove; besides these there are intervertebral FIG. 13. Tergal skeleton of and interambulacral longitudinal Asterias rubens. muscles. The ambulacral grooves a connecting pieces; 6, spine- i nii i i ,1 i &amp;lt; bearing plates. (After Gaudry.) are nearly filled with the tube-feet or pedicels, which have a nervous external and muscular internal layer, are usually cylindrical in form and furnished with terminal sucking-disks, and communicate by ducts passing through the ambulacral pores with vesicles lying above the ambulacral ossicles and opening into the ambulacral canal of the ray. In the common star-fish, Asterias (Asteracanthion) rube us the pores form a zig-zag line on each side of the ambulacral groove, and the pedicels passing through them thus come to be four- ranked (fig. 14). They are formed by notches or seini- FIG. 14. Asterias rubens. a, 4-ranked pedicels; 6, end of pedicel, magnified. pores incised one on the distal and the other on the oral surface of each ambulacral ossicle, and lying alternately external and internal to one another iu position on succes sive ossicles (fig. 15). The mouth, which is devoid of dentary apparatus, is situ ated in the middle of a membranous disk in the centre of the oral surface. It leads by a short gullet into the FIG. IS. Ossicles of ambulacral groove of Asterias tsfiiniph Tl p&amp;gt; rubens, viewed from above, a, pore for pedicel. stomach in most star-fishes is produced into five sac- culated prolongations (cardiac sacs); above these it con tracts, but again widens to form the pyloric sac; this gives rise to five tubes, which open out in each ray into a pair of parallel diverticula Laving numerous caecal dila tations, and connected by a mesentery with the antam- bulacral perisome. The pyloric sac in most cases leads into a short intestine terminating in an anus situated in the left posterior interradial space. In Astropecten, Ctenodiscus, and Luidia there is no anus. The madreporic tubercle is situated dorsally in the body disk, near one of the interradial angles; it is oval or slightly penta gonal in form, and the surface is marked with undulating grooves, and is finely perforated (fig. 16). In some genera FIG. 16. Antambulacral surface of Asterias rubens. a, madreporite; a, the same magnified; 6, anus. (Opkidiaster, Echinaster) there are several interradially placed madreporic tubercles. The doubly involuted madre poric canal is invested by the peritoneal membrane, which incloses a sinus, or &quot; heart,&quot; as it has been termed; it passes downwards into a pentagonal circum-oral ring which gives off the five radial canals occupying the upper most part of the ambulacral grooves. The circum-oral ring may or may not possess Polian vesicles. A dorsal or aboral ring has been described as communicating with the &quot; heart,&quot; and sending off interradial branches to the genital glands, the products of which, in the case of star fishes devoid of external genital apertures, it has been supposed they are the means of removing. The genital glands are racemose masses placed interradially in pairs; their processes sometimes extend a considerable distance into the arms. The nervous system consists mainly of a circular canal around the gullet, with five ambulacral trunks opening into it at their inner ends. The ambulacral neural trunk in each ray underlies a strong band of transverse fibres, by which it is separated from the am bulacral canal above. At the extremity of the ray the nerve terminates in an eye and its tentacle. The eyes are small processes of the ectoderm, having a convex surface or cornea containing a large number of simple, conical, pigmented ocelli. In the peritoneal cavity and ambulacral vessels is a watery fluid containing corpuscles. Respira tion appears to be effected by means of water supplied VII. So