244 ECHINODERMATA— ASTEROZOA phylum iv 



SuBPHYLUM B. Aster ozoa Leuckart.' 



Stemless Echinoderms loith depressed, pentagonal or star-shaped body, consisting 

 of a central disk and five or more rays (or "arms"). Mouth inferior and central in 

 position. Amhulacral tuhefeet restricted to the nnder surface of the rays. Internal 

 skeletal pieces of the ambulacra articulated together like vertebrae, or apposed like the 

 rafters of a pent-house. Integument coriaceous, strengthened by small, irregular, 

 loosely or firmly united calcareous plates, some of which bear spines, protuberances or 

 papillae, the whole constituting a covering showing the greatest diversity in details. 



The Asterozoa comprise the two classes of Asteroidea (Starfish) ; and 

 Ophiuroidea (Brittle Stars and Basket-fish). In both types the body consists 

 of a central disk containing the principal viscera, and giving oflf five or more 

 radiating processes or arms. The radiating ambulacral vessels are protected 

 by an internal skeleton consisting of a double row of calcareous bodies {ambul- 

 acral ossicles), the components of each pair being separated and movable to a 

 slight extent in the Asteroidea, but being welded together so as to form a 

 series of disks in the Ophiuroidea. The ambulacral grooves are open in the 

 Asteroidea ; but in the Ophiuroidea they are covered by dermal plates, and 

 the tube-feet project at the sides of the arms. The integumentary skeleton 

 sometimes appears leathery on the dorsal surface, but is generally strengthened 

 by calcareous plates or ossicles some of which usually bear spines or tubercles. 



Asterozoans are known as early as the Cambrian era, and have a continuous 

 history onward to the present time. They are of rather rare occurrence as fossils, 

 and are found chiefly in slaty, calcareous, or arenaceous strata which have been 

 deposited in shallow water. The Asterozoans are the most homogeneous and 

 most persistent type of all the Echinodermata. Both the Asteroidea and 

 Ophiuroidea are represented in the Ordovician and Silurian by well-differentiated 

 forms which do not differ materially from those now living. The only notice- 

 able difference is that many of the Paleozoic Asterozoans exhibit an alternate 

 arrangement of the ambulacral ossicles, Avhile in all Recent species these are in 

 a double row, with the ends directly apposed. 



Whether this more or less disjunct and alternating condition of the 

 vertebi'al ossicles in Paleozoic Asterozoa is really a primitive feature of 

 fundamental importance must still be considered an open question. It is 

 quite possible that the apparent alternation in Paleozoic starfishes is due 

 to conditions of preservation, or if not, is an inconstant and insignificant 



^ Literature: Mullcr, J., and TroscheJ, F. H., System der Asteriden. Brunswick, 1842. — 

 Forbes, E., Monograph of the Echinodermata of the British Tertiaries. Palaeont. Soc, 1852. — 

 Billings, E., Figures and Descriptions of Canadian Organic Remains. Geol. Survey Canada, Decade 

 iii., 1858. — Wright,^!., Monograph on the British Fossil Echinodermata of the Oolitic Formations, 

 vol. ii., Asteroidea and Ophiuroidea. Palaeont. Soc, 1863-80. — Hall, J., Twentieth Report on the 

 New York State Cabinet, 1868. — Quenstedt, F. A., Petrefactenkunde Deutschlands, vol. iv., 1874- 

 1876. — Liulwig, H., Morphologische Studien an Echinodermeu. Leipzrc, 1877-79. — Newnimjr, 31., 

 Morphologische Studien iiher fossile Echinodermen. Sitzungsber. Akad. Wiss. Wien, vol. Ixxxiv. 

 1881. — Carpenter, P. //., Minute Anatomy of the Brachiate Echinodermata. Quart. Journ. Micro- 

 scop. Sci., 1881. — Sturtz, B., Beitriige zur Kenntniss paliiozoischer Seesterne. Palaeontographica, 

 vols, xxxii. and xxxvi., 1886, 1890. — Idem, Uber versteinerte und lebende Seesterne. Verhandl. 

 (1. naturhist. Vereins Rheinlande, Westpiialen, etc., 5th ser. vol. x., 1892. — Sladen, W. P., and 

 Spencer, W. K., Monograph on British Fossil Echinodermata from the Cretaceous Formations, vol. ii. 

 Asteroidea and Ophiuroidea. Palaeoutogr. Soc, 1891-1908. — Oregory, J. IF., The Stelleroidea, in 

 Lankester's Treatise on Zoology, Part iii., 1900. — .Taekel, 0., Asteriden und Ophiuriden aus dem 

 Silur Bohmens. Zeitschr. Deutsch. Geol. Ges., 1903, vol. \y. ^Bather, F. A., Guide to Fossil In- 

 vertebrates, etc. British Mus. Publ. , 1907. — Schondorf, F., tjber einige Oi)hiurideu und Asteriden 

 des englischen Silur, etc. Jahrb. Nassauischen Ver. Naturk., Jlirg. 63, 1910. 



