ECHINODKKMATA 87 



tides of calcium carbonate. This stone canal connects in turn 

 with a circular canal which runs around the mouth, and pos 

 sesses on its wall several small sacs, the Polian vesicles. From 

 this circular or ring canal five branches are given off, one ol 

 which lies along the median line of each radius, and gives ofi 

 numerous short, lateral branches, which connect with the tube 

 feet, or ambulacra] feet. Each foot terminates within the 

 body in a sacular enlargement, called the ampulla (Fig. 74). 

 The sea water, which has free access to this system of tubes 

 through the madreporic plate, can be forced into the tube feet, 

 which thus become elongated ; and, coming in contact with some 

 foreign body, they attach themselves to it by their suckerlike 

 ends, and then shorten by the contraction of the muscles in 

 their walls, forcing the water into the ampulla", while the ani- 

 mal is drawn toward the object to which the foot is attached. 

 In some Echinodermata, the tube feet do not function as organs 

 of locomotion, but are modified into feelers or tentacles. 



The sexes are separate in almost all species of this type, but 

 in a few cases the animals are hermaphroditic ; even here, how- 

 ever, the individual does not produce ova and spermatozoa at 

 the same time, but while young is exclusively of one sex, either 

 male or female, and as it grows older becomes exclusively of 

 the opposite sex. There are five classes of living Echinoder- 

 mata, and two which are exclusively fossil. 



CLASS I. CRINOIDEA 



The Crinoidea (Gr. xpivov, lily, and etSo?, form) are known 

 popularly as the sea lilies or the stone lilies, or sometimes as the 

 feather stars. These animals were once very abundant, for we 

 find nearly four times as many fossil species as there are species 

 living to-day, and their remains extend as far back as the Cam- 

 brian. They live almost exclusively in deep water, so that they 

 can be obtained only by dredging. Almost all are provided 

 with a stalk, consisting of calcareous plates, piled one upon 

 the other (Fig. 75). This stalk is not very long, — in some 

 cases it may be half a meter or more in length, but among the 

 fossils it has been found from eighteen to twenty meters long. 

 In one group of living Crinoidea there is no stalk in the adult 



