206 ECHINODERMATA—ASTEROZOA SUB-BRANCH II 
plates. Each of the arms is traversed on the under or oral side by a broad 
and deep furrow, which tapers gradually in passing from the mouth to the 
tip of the arm, where it is terminated by a simple grooved plate (Fig. 333) 
called the ocular plate. The roof of each ambulacral furrow is formed by two 
rows of rafter-like, rather elongate, ambulacral 
ossicles, the inner ends of which are held 
together by muscles (Figs. 334, 335). Run- 
ning along the centre of the groove on its 
Fra. 334, ventral side are placed in succession the radial 
iiss ae Detached ambu- water-tube, blood-vessel, and nerve cord. 
Ocular plates of  lacral ossicle of Pen- 5 : 
Pentagonoster(?)from  tagonaster (2) from These are all homologous with the like- 
Beaters ie or Seiten it named organs of Ophiuroids. 
The form of the ambulacral ossicles 
varies in different genera. In all recent forms the ends are directly apposed 
against one another in the median line of the ambulacral grooves; but in 
Palaeozoic forms they are arranged in alternate rows, and are inclined towards 
one another at a very small angle. Each pair of ambulacral plates is ex- 
cavated at the sides, so as to give rise by their apposition to a series of small 
A B 


Fic. 336. 
Fic. 335. Asteriaus rubens, Linn. Recent; 
Astropecten aurantiacus, Linn. sp. Recent ; German Ocean. Enlarged vertical 
Mediterranean. Enlarged vertical section of section of one of the arms. am, Am- 
one of the arms. am, Ambulacral ossicles ; bulacral ossicles; ad, Adambulacral 
ad, Adambulacral plate ; mv, Infero-marginal plates; mv, Infero-marginal plates ; 
plate ; md, Supero-marginal plate; 7, Super- a, Radiating water-tube ; b, Ampullae ; 
ambulacral plate. p, Tube feet. 
apertures, through which the distensible tube feet or pedicels are emitted. 
The latter are the downward prolongations of lateral branches given off by 
the radial ambulacral vessel; the upward prolongations of the same form 
small sacs called ampullae, by means of which water is forced into the 
tube feet. 
The lower ends of the ambulacral ossicles rest against a series of 
adambulacral plates, and in many forms these are bounded in turn by large 
marginal plates (Fig. 335). Intermediate plates are those which are inserted 
between the infero-marginal plates and the adambulacral plates. By the 
term dorsal plates are understood all calcareous bodies occurring on the dorsal 
side of the body. 
Perfectly preserved Star-fishes are known only from a few localities, 
such as Bundenbach in Rhenish Prussia, the usual mode of occurrence being 
in the form of moulds or detached plates. The earliest forms are found in 
Cambrian rocks. 
