ASTEROZOA 199 



SUB-BRANCH II. Asterozoa. Leuckart. 1 



iilessEchiiwIi-rni* > < I th depressed , ////<///"/////, <>, star-shaped body, consisting of 

 ,t central '//.</. "///.///>' <>r mmr /V///.S (,>,- " iinns"). Mouth inferior and central in 

 jfixffi'in. . liiibnlnrnil f,i/,,'fnf rrdriHnl to the under surface of the rays. Internal 

 th'lt'tnn, pieces of the ///W//ov> nrf/mlafnl together like vertebrae, or apposed lib 1 ///> 

 ruff* rs of a pent-house. Integument coriaceous, strengthened by small, irregular, 

 loosely tut H<;/ mlcareous plates, some of which bear spines, bristles, protuberances, or 

 jHi/'/ilae ; the whole constituting a covering of the most varied kind. 



The Asterozoa comprise the two classes of Asteroidea, or Star-fishes ; and 

 <>/>/// ii roidea, 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 

 ly ;in internal skeleton consisting of a double row of calcareous bodies (am- 

 Imhn-ral 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 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 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 Literatim- : 



M;HI,'r, ,/., anil Troschel, F. //., System der Asteriden. Braunschweig, 1842. 

 7- '"//, 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. 

 MVi/////. 7*., .Monograph on the British Fossil Echinodermata of the Oolitic Formations, vol. II., 



Asteroidea and Ophiuroidea (Palaeont. Soc.), 1863-80. 

 Hull, J., Twentieth Report on the New York State Cabinet, 1868. 

 V".sY,v//', F. A., Petrefactenkunde Deutschlands, Band. IV., 1874-76. 

 /./'(//'/</, //., Morphologische Studien an Echinodermen. Leipzic, 1877-79. 

 .Y' "///"//>, .I/., .Morphologische Studien iiber fossile Echinodermen (Sitzungsber. Wien. Akad. 



I. XXX IV.), 1881. 

 Carpenter, /'. 11.. Minute Anatomy of the Brachiate Echinodermata (Quart. Journ. Microscop. Soc., 



XXL), 1881. 

 Sttoix, /!., Beitrage zur Kenntniss palaeozoischer Seesterne (Palaeontographica, XXXII. and XXXVI. ), 



1886, 1890. 

 Sfdrtz, B., Ueber versteiuerte und lebende Seesterne (Verhandl. d. naturhist. Vereins Rheinlande, 



Westphalen, etc., 5th Folge, Bd. X.), 1892. 



