CRINOIDS. 



141 



the stem are attached a variable number of plates, normally including one or more 

 basals, and three or more sets of radials, which, with interradials, etc., make up a cup 

 or calyx in the hollow 

 of which the internal 

 organs of the animal 

 are accommodated. 

 From the edge of the 

 calyx spring a number 

 of jointed arms, usually 

 five, these again divide 

 once, twice, or more 

 times, and each' arm is 

 furnished with pinnules 

 as a feather is set with 

 barbs. The ambulacral 

 feet are situated in the 

 furrows of the calyx and 

 along the arms. No 

 other echinoderms are 

 fixed at any period of 

 their life-history ; in no 

 others do the arms sub- 

 divide into pinnules, 

 and in no others is the 

 madreporic plate ab- 

 sent, though in most 

 holothurians it is in- 

 ternal. The mouth is 

 situated in the centre 

 of the upper side of the 

 calyx, instead of in the 

 centre of the lower side 

 as is the case in star- 

 fishes and sea-urchins, 

 while the anus is placed 

 on a conical projection 

 between the bases of 

 two of the arms. It is 

 thus probable that the 

 upper surface of a 

 orinoid is homologous 

 with the lower surface 

 of a star-fish or sear 

 urchin. The oesopha- 

 gus is short, and the in- 

 testinal canal is more or less coiled in its passage to the anal extremity. The 

 mouth does not contain any masticatory apparatus, comparable with the so-called 

 ' jaws ' and ' teeth ' of sea-urchins and serpent-stars. The ujjper surface of the 



FlGr. 127. — Antedon on tlie tube of a ■worm. 



