CRINOIDS. 301 



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 

 articulation 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 solidification 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 provided with five paddles. In Uintacrinus the ten arm-branches reached the 

 enormous length of 3 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 the free forms is effected by the raising and depressing of 

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

 nervous system that has its headquarters at the bottom of the cup. This 

 swimming has been observed in both Antedon and Actinometra kept in an 

 aquarium. As a rule, however, these animals remain attached by their cirri to 

 rocks, to the bottom ooze, to seaweeds, or to other marine animals. In this 

 position the arms are outspread, and the small branches or pinnules that line their 

 sides are kept slightly waving. If the water be ruffled, the first impulse of the 

 crinoid is to flatten its arms out suddenly and to hold on to the rock or other 

 object with its pinnules. The pinnules of an Antedon can be bent in any direc- 



