3268 ECHINODERMS 



become a little unsymmetrical and bent over In one direction; which may, perhaps, 

 be accounted for by their life on reefs, where food is brought to them by currents 

 flowing only in one direction. 



Next to the stem, the most characteristic structures of a crinoid are its 

 arms. Each arm starts from one of the five plates that form the uppermost circlet 

 in the cup. The arms are said to be radial in position, and those plates from 

 which they start are specially distinguished as the " radials." In many forms, such 

 as Cyathocrinus, the upper edge of each radial is notched by a horseshoe-shaped 

 facet, provided with a transverse fulcral ridge and muscles, so that a regular artic- 

 ulation is formed for the working of the arm up and down. In such forms 

 known as Inadunata the arms are quite separate from one another and are easily 

 distinguished from the plates that compose the cup. But in the forms to which the 

 names Flexibilia and Camerata are applied, smaller plates are developed in the spaces 

 or interradii between the arms, and these additional plates bind the arms together 

 and so incorporate them in the walls of the cup. A crinoid of this kind, such as 

 Actinocrinus or Uintacrinus, has therefore a much larger body than a Cyathocrinus 

 or Pentacrinus. Sometimes the arms form part of the cup without the intercalation 

 of interbrachial plates, while yet other plates may be developed between the forks 

 of the arms themselves. In the Flexibilia the plates that form the cup are rather 

 loosely joined to one another, so that there is some play between them; the arms 

 also have much power of motion. In the Camerata the plates are more firmly 

 united, and additional fixity is given to the cup by the strengthening and solidifica- 

 tion of the upper surface around the mouth. In the Inadunata and Flexibilia the 

 grooves on the inner surface of the arms, which convey food to the mouth, pass 

 over this upper surface of the cup, and are merely protected by the ordinary small 

 plates that can be opened or shut down over them. But in the Camerata the plates 

 of this upper surface of the cup have become so thick and welded together, that the 

 grooves are no longer open, and even in some cases have been pressed down beneath 

 the surface, underneath which they form regular tunnels. The mouth too is no 

 longer visible on the upper surface. Crinoids of this type were most abundant in 

 the Carboniferous period, and it is to a large extent their remains that make up the 

 masses of Derbyshire marble. 



Among curious modifications of arm structure may be mentioned the Silurian 

 Crotalocrinus. Here the arms are forked many times, but all the separate branches 

 are joined together at their sides, so that the arms when outspread form a single 

 net. In Petalocrinus of the same age this process has been carried so far that the 

 branches of each arm are solidly fixed together, and the crinoid appears to be pro- 

 vided with five paddles. In Uintacrinus the ten arm branches reached the enormous 

 length of three feet, and seem to have been capable of movement in various 

 directions, so that the swimming powers of the crinoid must have been greatly 

 enhanced. Saccocoma, which lived in the still lagoons where the Solenhofen litho- 

 graphic stone was deposited, had a very light body and long, fine arms, provided 

 with flat oar-like processes. 



The locomotion of Ijie free forms is effected by the raising and depressing of 

 alternate arms, and the movements of these arms are correlated by the peculiar 



