CRIXOIDS. 



141 



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■iff 



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

 basals, and three or more sets of radials, which, with interi-adials, etc., make uji a cuj) 

 or calyx in the hollow ,r _ 

 of which the internal f 

 organs of the animal i 



a r e accommodated, f 

 From the edge of the ; - ,,\ 



calyx spring a number ^ '^--; 



of jointed arms, usually : ~ , 



live, these again divide 

 once, twice, or more 

 times, f.nd 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 arras. No 

 other echinoderms are 

 lixed 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 ig placed 

 on a conical projection 

 between the bases of 

 two of the arms. It is 

 thus probable that the 

 u]iper surface of a 

 irinoid is homologous 

 with the lower surface 

 of a star-fish or sea- 

 urchin. The oesopha- 

 gus is short, and the in- 

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

 mouth does not contain any masticatory aj)paratus, comparable with the so-called 

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



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1-7, — Antcdon on the tube of a worm. 



