136 



L O WER IN VER TEBRA TES. 



find a madreporic body like that of the star-fish. It is thus evident that these rows 

 of plates correspond to the ambulacral areas of the star-fish, and we can readily see 

 that were we to bend the arms of the latter form upwards, so as to form a ball (most 





Fig. 125. — Anatomy of star-fish; a, duct from liver to stomach; 6, liver; c. niaJreporic body; 

 jt ampuUic; i, ambulacral plates: ?«, inler-ainbulacral plates. 



of the upper surface disapjiearing during the operation), the star-fish would be con- 

 verted into a sea-urchin. 



The typical number of similar parts (ambulacral and inter-ambulacral areas) which 

 go to msike up Echinoderm is five, though frequently this number is exceeded. This 

 radial arrangement is also seen in some of the iiiterii;d organs, but it is not visible in 



