194 PHYLUM ECHINODERMATA. 



Class OPHIUROIDEA * 



Brachiate Echinoderms with the body flattened in the or-anal 

 axis. The arms are sharply marked off from the disc and are 

 without an ambulacral groove. The madreporite is on the oral 

 surface. 



The Ophiurids are brachiate Echinoderms, and the arms 

 which are very rarely more than five in number (e.g. Ophioglypha 

 hexactis with 6 arms, Ophiocantha vivipara 6 or 7 arms, 0. 

 anomala and nodosa 6 arms etc.) are sharply marked off from 

 the disc. The ambulacral grooves are absent or so slightly 

 marked as not to be noticeable, and in most> forms the ambula- 

 cral nerve tracts are separated from the ectoderm by calcareous 

 plates. The generative organs usually open into special pockets 

 developed on the oral side of the disc at the base of the arms and 

 called the genital bursae. The alimentary canal is without an 

 anus and is not prolonged into the arms. The water-pore (or 

 pores) is oh the oral surface of the disc, and interradial in posi- 

 tion. There is an axial sinus, axial organ, and generative 

 rachis. The tissue of the so-called vascular system is feebly 

 developed, and the tube-feet are without ampullae. As 

 stated above the arms are rarely more than five in number, but 

 in the Cladophiurae they may be much branched (Fig. 146). 



The integument is not ciliated, and except in the Cladophiurae 

 and perhaps some Streptophiuridae the ectoderm is not present 

 as a layer distinct from the dermis. The dermis is without 

 muscles, but in all Ophiuroids, except the Cladophiurae, is richly 

 provided with calcareous plates. As a general rule these plates 

 form a complete dermal armour which may or may not be 



* A. Ljungman, Ophiuridea viventia hue usque cognita, Stockholm, 

 1867. H. Ludwig, Trichaster elegans, Z.f.w.Z., 31, 1878. Id., Das 

 Mundskelet der Asterien u. Ophiuren, Ibid., 32, 1879. Id., Zur Ent. d. 

 Ophiurenskeletes, Ibid., 36, 1882. Id. Ophiopteron elegans etc., Ibid., 

 47, 1888. Th. Lyman, Report on the Ophiuridea, Challenger Reports, 

 5, 1882. Id., Ophiuridae and Astrophytidae Illustrated Catalogue of the 

 Museum of Comp. Anat. of Harvard College, I. Cambridge, Mass., 1865. 

 L. Cuenot, Etudes anatomiques sur les Ophiures, Arch. Zool. Exper. 

 et gen (2), 6, 1888, p. 33. E. W. MacBride, Development of the 

 genital organs, etc., in Amphiura squamata, Q.J.M.8., 34, 1893. F. 

 Jeffrey Bell, A Contribution to the Classification of Ophiuroids, etc., 

 Proc. Zool. Soc., 1892, p. 175. J. W. Gregory, Classification of the 

 palaeozoic Echinoderms (Ophiuroidea), Proc. Zool. Soc., 1896, p. 1028. 

 Liitken et Mortensen, The Ophiuridae, Mem. Mus. Harvard College, 23, 

 1899. O. Hamann, Ophiuroidea, in Bronrfs Thierreich, 1900-1901. 



