CRINOIDS. 
30 1 
as Cycithocrinus, 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 
Cycithocrinus 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. I 11 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. Sciccocoma, 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 fiat 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- 
