I rise bv 



ASTEROIDEA 



247 



rise by their apposition to a series of small apertures, through which the 

 distensible tube feet or pedicels are emitted. The latter are the downward 



md 



ad^ mir. 



Fig. 349. 



Astropecten aurantiacus (Linn.). Recent; 

 Mediterranean. Enlarged vertical section of 

 one of the arms. am, Ambulacral ossicles ; 

 ad, Adambulacral i)late ; mv, Infero-marginal 

 plate ; md, Supero-marginal plate ; i, Super- 

 ambulacral plate. 



aa. 



FiQ. 350. 



Asterias ruhens Linnaeus, Recent ; 

 German Ocean. Enlarged vertical 

 section of one of the arms. am, Am- 

 bnlacral ossicles ; ad, Adambulacral 

 plates ; mv, Infero - marginal plates ; 

 a, Radiating water-tube ; b, Ampullae ; 

 p, Tube-feet. 



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. 349). 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 Starfishes 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. 



There is no generally accepted Classification of the Starfishes. Not only 

 do specialists disagree as to the Orders and families, but tjiere is the widest 

 divergence of opinion as to the principles upon which the Classification should 

 be based. Unfortunately none of the zoologists who have in recent years 

 attempted to formulate a Classification for the group, except Sladen,.has taken 

 fossils into account, and even Sladen was inclined arbitrarily to separate 

 Paleozoic and Recent forms. The latest authority, Fisher, accepts three 

 Orders but does not consider their limits as satisfactorily determined. The 

 study of Recent forms has shown that the characters of the tube-feet, 

 reproductive organs and other soft parts are of real importance in determin- 

 ing family limits, and that the pedicellariae are possibly of even ordinal 

 importance, hence it is exceedingly difficult to intercalate fossil Starfishes in a 

 Classification of the living forms. As has been suggested above, Gregory is 

 very possibly correct in his view that the alternation of the ambulacral 

 ossicles cannot be considered of fundamental importance, but may often be 

 only a result of pressure during fossilisation. It certainly ought not to be 

 used to isolate all Paleozoic forms, or most of them, in a class by themselves. 

 One character upon which stress was first laid by Sladen has come to be 

 generally regarded as of fundamental importance, i.e. the size and appearance 

 of the marginal plates. The genera in which these plates are large and 

 conspicuous have the papulae confined to the space bounded by the upper 

 series or supero-marginals ; and this group, called Phanerozonia by Sladen, is 

 now quite generally accepted although its exact limits, at least among living 



