136 



LOWER INVERTEBRATES. 



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

 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; b, liver: c, mailreporio body; 

 j, ampullae; Z, ambulacral plates: m, liiter-ambulacral plates. 



of the upper surface disappearing 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 mske up Echinoderm is five, though frequently this number is exceeded. This 

 radial arrangement is also seen in some of the internal organs, but it is not visible in 



