Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/657

ECHINODERMATA 635 are usually four interradially placed Polian vesicles. The necks of the Polian vesicles and the ainbulacral ring give off the diverticula termed by Simrock vasa ambulacralia cavi. From the ambulacral ring proceed^ the five radial canals between the superambulacral plates and the axial ossicles. In front of each ossicle they give off right and left branches to the pedicels. These are tentacle-like, devoid of basal vesicles, and, except in the Euryalidw, pass out through openings between the superambulacral and 1 iteral plates. The nervous system consists of an oral ring, which supplies a branch to each arm, running superficial to its radial ambulacral canal. Between the nerve and the latter is a neural canal. The genital organs are pairs of racemose glands attached to the inner dorsal surface of the disk ; their products are shed into the perivisceral cavity, whence they make their way through the genital clefts between the origins of the arms. Hermaphrodism lias been observed in the species OpliioUpis squa- inata; and in some genera, as Ophiocoma and Ophiactis, scissiparous reproduction occurs. According to Liitken, this at an early age is more especially exhibited by the six-rayed forms. In certain cases development takes place within the egg, without any free pseud- embryonic stage ; but most generally metamorphosis from a pluteiform larva takes place. The bilaterally symmetrical skele ton of the pluteus consists of eight radially diverging calcareous rods. The development of the embryo commences with the pro duction of two cylindrical solid bodies, one on each side of the gullet, which form cellular blastemic masses, one behind and another in front of the stomach, and a third to the left of the pseudostome. The ambulacral system of the adult is developed from the last of these, which unites with the mass in front of the stomach to form the ventral portion of the body, whilst the posterior mass furnishes the dorsal portion. The Ophiuridea may be classified as follows : Sub-order I. OPHIURIDJE. Arms unbranched ; ambulacral furrows covered with plates; genital clefts ordinarily five; habit creeping. A. Oral clefts armed. (i.) No papillae, angulares. OPIIIODERMATID.E. Buccal scutes trigonal ; disk granulated. Ex. Ophiura (Ophioderma), OpMo- chceta, Ophiqpsammus. OpniOLEi iu^E. Buccal scutes pentagonal ; disk scales naked. Ex. Ophiolepis, Ophioceramis, Ophiopus. AMPHIURID.E. Disk rugged and scaly ; ray-plates spinous. Ex. Ampliiura, Ophiacantha, OpMopholis, Ophiostigma, Ophiactis. OPHIOMYXID.E. Disk naked ; rays clothed with soft integument. Ex. Ophiomyxa, Opliioscolex. (ii.) Papillae angulares present. OPHIOCOMIDJE. Disk covered with solid plates. Ex. Ophiocoma, Ophiomastix, Ophiarthrum. B. Oral clefts unarmed. OPHIOTHRldD.ffi. Radial plates very large. Ex. Ophiothrix, Ophiocnemis, Ophiogymna. Sub-order II. EURYALID/E. Arms simple or ramified, and capable of being rolled up towards the mouth ; ambulacral furrows covered by soft integument. Spines are not present, but there are tufts of papillae on the ventral surface of the arms ; genital clefts ten. ASTROPHYTID.E. Astrophyton, Trichaster, Aster- onyx, Astcromorpha, Asteroporpa. Distribution in time of Ophiuridea. The following Paleozoic genera are commonly referred to the Ophiuridea : Protaster (Lower Silurian); Palwodiscus, Acroura, and Eudadia (Upper Silurian); Eugaster (Devonian of New York). Ophiura (?) occurs in the Carboniferous Limestone of Russia. In the Muschelkalk occur Aipidura and Aplocoma, and in higher Secondary strata Opliio- dsrma, Ophiocoma, Amphiura, and other genera. The Ophiuridea and Asteridea possess, in a marked degree, the power exhibited by the whole of the Echinodermata of reproducing lost portions of the body. The former have re ceived the name of "Brittle-stars" on account of the remark able facility with which species of the genus Ophiocoma not merely cast away their arms entire, but, at will, rapidly break them into little pieces. The same property has been noticed in the genus Luidia among the star-fishes, i i the Crinoidean genus Comatu a, and the Synaptidx among the Holothuridea. Writing of a species of Luidia, Prof. E. Forbes remarks : " The first time I ever took one of these creatures I succeeded in getting it into the boat entire. Never having seen one before, and quite un conscious of its suicidal powers, I spread it out on a rowing bench, the better to admire its form and colours. On attempting to remove it for preservation, to my horror and disappointment, I found only an assemblage of rejected members. My conservative endeavours were all neutralized by its destructive exertions, and it is now badly represented in my cabinet by a diskless arm and an armless disk." Major Fred. H. Lang relates (Nature, Oct. 12, 1876), that during a dredging expedition in Torbay, presuming on the fact that as a rule he could take up the specimens of Comatida rosacea and Ophiocoma rosula he had captured without occasioning their dis memberment, he " put about a hundred of the two sorts into a sponge-bag; but this was asking too much of them ; " for on reaching home he found " that both Feather-stars and Brittle-stars had converted themselves into a mass of mince-meat ! It would have been difficult to find a single portion of an arm a quarter of an inch long." Order 1 7. CiUNOlDEA. The body in the Crinoidea is cup-shaped or bursiform, and its base always in the young state and usually in the adult is attached by the apical pole either directly or, as more commonly, by means of a calca reous stem to sub marine objects. The inferior or dorsal wall of the body or calyx is formed of poly gonal plates arti culated by their edges, and the su perior or ventral face or disk, which may be either flat or arched, is form ed either by a perisomal mem brane, occasionally strengthened with scattered calcifica tions, or, as in the Tessdata, by regularly arranged plates, tegminalia, resembling those of the inferior wall. At the border of the calyx are 2-1 8, usually 5, arms or brachia, which are movable, and can be closed together over the oral disk (fig. 22). Between them, commonly in the centre of the disk, is the mouth, and near it, in one of the interradial spaces, is the anus. It has been shown by Sir "Wyville Thomson, (Phil. Trans, vol. civ. pt. 2), that the skeleton of Antedon rosaccus may be divided into two systems of plates, the radial and the perisomatic, the former including the articuli of the stem, the centro-dorsal plate, the radial plate, and the joints of the arms and pinnules, and the latter the basal, oral, and anal plates, and the interradial and other plates or spieulae developed in the disk-membrane. " The body of FIG. 21. Pentacrinus caput-Medusa?. Guttard.)

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