252 



ECHINODERMATA— ASTEROZOA 



PHYLÜM IV 



Cryptozonate Asteroids are very rare. The Recent 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 ruhens Linn. It is stränge that no good 

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



Olass 2. OPHIUROIDEA Gray. Brittle Stars.i 



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

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

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

 calcareous Joint s, the elements of which are usually fused to form ^^ vertebral ossicles" 

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

 ambulacral grooves. Madreporite constantly on the actinal {oral) side of the disk. 



AB D 



Fig. 358. 

 A, Vertical section of an Ophiuran arm. w, Vertebral ossicle ; a, Ambulacral vessel, witli side-branches lead- 

 iiig into the tube-feet ; h, Blood-vessel ; tc, 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 ; i/, 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 



1 Literature : Lütken, C. F., Additamenta ad historiam Ophiuridarum. Köngl. dan. Vidensk. 

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

 Mus. Comp. Zool. Cambridge, Nos. i.-iii., 1865. — Ludwig, IL, Beiträge zur Anatomie der 

 Ophiuren. Zeitschr. für wissensch. Zool., vols. xxxi., xxxiv., 1878-80. — Ludwig, H., Morphologische 

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

 Expedition, Zoology, vol. v., 1882. — Picard, K., Über 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 

 ofthe Palaeozoic Echinoderms of the gTOup Ophiuroidea. Proc. Zool. Soc, London, 1896. — Sollas, 

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

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

 reichs, 1901. — Strassen, 0. zur, Zur Morphologie des Mundskelettes der Ophiuriden. Zool. Anz., 

 1901, vol. xxiv. — Jaekd, 0., Ästenden und Ophiuriden aus dem Silur Böhmens. Zeitschr. Deutsch. 

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

 Inst., 1909, vol. viii. — Sollas, L B. J. and W. J., Lapworthura : A typical Brittle-star of the 

 Silurian Age. Phil. Trans., 1912, vol. ccii. 



