252 



ECHINODERMATA— ASTEROZOA 



PHYLUM IV 



Cryptozonate Asteroids are very rare. The Kecent genus Solaster Forbes is 

 represented by a species with numerous arms in the Great Oolite of England. 

 Two important families of Recent Cryptozonia are the Echinasteridae and 

 the Asteriidae. A species of Echinaster M. and T. has been described from 

 the Neocomian, and Forbes thought he found in the Red Crag of England 

 remains of the now common Asterias rubens Linn. It is strange that no good 

 evidence has been found of the occurrence of Asterias in Tertiary strata. 



Class 2. OPHIUROIDEA Gray. Brittle Stars.i 



Asterozoans having a more or less sharply defined central disk containing a simple 

 digestive cavity which does not radiate into the slender rounded arms, and has no anal 

 opening. Reproductive organs confined to the disk. Arms with an axis composed of 

 calcareous joints, the elements of which are usually fused to forin "vertebral ossicles," 

 encased with plates or covered with a leathery skin, and very rarely ivith open 

 ambidacral grooves. Madreporite constantly on the actinal (oral) side of the disk. 



y Fir;. 358. 



A, Vortical section of an Opliiuran ami. w, Vertebral ossicle ; «, Anibulacral vessel, with side-branches lead- 

 ing into the tube-feet ; h. Blood-vessel ; ii, Nerve-cord ; v, Ventral or lower arm-plate; I, Side-plates ; d, Dorsal 

 plate. B, Vertebral ossicle, seen from the inward side, with surrounding arm-plates. C, Row of vertebral 

 ossicles viewed from the side, and slightly enlarged ; x, Apertures where the branches of the ambulacral vessel 

 enter and emerge from the arm-bones ; y, Depressions for the insertion of intravertebral muscles. D, Mouth- 

 frame of an Ophiuran, with the proximal vertebral ossicles. The heavy lines bordering the arms represent the 

 genital slits ; the dark pentagon in the centre marks the course of the nerve-ring. 



Ophiuroids are distinguished from the typical Starfishes by their cylin- 

 drical flexible arms, which are sharply separated from the central disk, and 



^ Literature : Liltken, C. F., Additamenta ad liistoriam Ophiuridarum. Kougl. dan. Vidensk. 

 Selskabs Skrifter, v. and viii., 1858-69. — Lyman, T., Opbinridae and Astrophytidae. Illustr. Cat. 

 Mus. Comp. Zool. Cambridge, Nos. i.-iii., 1865. — Ludwig, II., Beitriige zur Anatoniie der 

 Ophiuren. Zeitschr. fiir wissenscli. Zool., vols. x.xxi., xxxiv., 1878-80. — Ludwig, H., Morphologische 

 Studien an Echinodermen. Leipzic, 1877-79.— Lymcpi, T., Report on the Ophiuroidea. Challenger 

 Expedition, Zoology, vol. v., 1882. — Picard, K., tjber Ophiuren aus dem oberen Muschelkalk. 

 Zeitschr. deutsch. geol. Gesellsch., vol. xxxviii., 1886. — Boehm, G., Beitrag zur Kenntniss fossiler 

 Ophiuren. Berichte naturf. Gesellsch., Freiburg, v., 1889. — Gregory, J. W., On the classification 

 of the Palaeozoic Ecliinodernis of the group Ophiuroidea. Proc. Zool. Soc, London, 1896. — SoUas, 

 W. J., On Silurian Echinoidea and Ophiuroidea. Quart. Journ. Geol. Soc. , 1899, vol. Iv. — Ilainann, 

 0., Die Schlaugensterne. Buch iii., Abt. 3, Bd. 2, of Bronn's Klassen und Ordnuugen des Tier- 

 reich.s, 1901. — Strassen, 0. zur, Zur Morphologie des Mundskelettes der Ophiurideu. Zool. Anz., 

 1901, vol. xxiv. — Jnekd, 0., Asteriden und Ophiurideu aus deni Silur Bohmens. Zeitschr. Deutsch. 

 Geol. Ges., 1903, vol. Iv. — Parks, W. A., Notes on the Ophiuran genus Protaster. Trans. Canad. 

 In.st., 1909, vol. vhi.—SoUas, I. B.J. and W. J., Lapworthura : A typical Brittle-star of the 



Silurian Age. 



Phil. Trans., 1912, vol. ccii. 



