SUB-BRANCH II ASTEROZOA 199 
Sup-Brancu Il. Asterozoa. Leuckart.' 
Stemless Echinoderms with depressed, pentagonal, or star-shaped body, consisting of 
w central disk and five or more rays (or “ arms”). Mouth inferior and central in 
position. Ambulacral tube feet restricted to the under surface of the rays. Internal 
skeleton pieces of the ambulacra articulated together like vertebrae, or apposed like the 
rafters of a pent-house. Integument coriaceous, strengthened by small, irregular, 
loosely united calcareous plates, some of which bear spines, bristles, protuberances, or 
papillae ; the whole constituting a covering of the most varied kind. 
The Asterozoa comprise the two classes of Asteroidea, or Star-fishes ; and 
Ophiuroidea, or Brittle-stars and Sand-stars. In both types the body consists 
of a central disk containing the principal viscera, and giving off 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 (aim- 
bulacral ossicles), the components of each pair being separated and movable to 
a slight extent in the <Asteroidea, but are 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 
appears leathery on the dorsal surface, but is generally strengthened by 
innumerable small calcareous bodies, on some of which are placed movable 
spines or bristles. 
Fossil Asterozoans are found as far back as the Cambrian era, and are 
represented continuously down to the present time. They are of rather 
uncommon occurrence, and are found, as a rule, in caleareous 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 noticeable difference is that many of the Palaeozoic 
Asterozoans exhibit an alternate arrangement of the ambulacral ossicles, 
these being developed in all recent species in a double row, with the ends 
directly apposed. The more or less disjunct condition of the vertebral 
ossicles in Palaeozoic Ophiuroids must be regarded as an embryonic character. 
1 Literature : 
Miller, J., and Troschel, F. H., System der Asteriden. Braunschweig, 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 ITI.), 1858. 
Wright, T., Monograph on the British Fossil Echinodermata of the Oolitic Formations, vol. IL., 
Asteroidea and Ophiuroidea (Palaeont. Soc.), 1863-80. 
Hall, J.. Twentieth Report on the New York State Cabinet, 1868. 
Quenstedt, F. A., Petrefactenkunde Deutschlands, Band. IV., 1874-76. 
Ludwig, H., Morphologische Studien an Echinodermen. Leipzic, 1877-79. 
Neumayr, M., Morphologische Studien iiber fossile Echinodermen (Sitzungsber. Wien. Akad. 
LXXXIV.), 1881. 
Carpenter, P. H., Minute Anatomy of the Brachiate Echinodermata (Quart. Journ. Microscop. Soc., 
XXI.), 1881. 
Stirtz, B., Beitriige zur Kenntuiss palaeozoischer Seesterne (Palaeontographica, XXXII. and XXXVI), 
1886, 1890. 
Stirtz, B., Ueber versteinerte und lebende Seesterne (Verhandl. d. naturhist. Vereins Rheinlande, 
Westphalen, etc., 5th Folge, Bd. X.), 1892. 
