STAR-FISH AND SEA-UllCIIINS. 2/3 



mobile rays. J\[icl\vay in tlie series we have Astro- 

 pecten and the common Star-fish, where the rays 

 are flexible and mobile, though not nearly so much 

 so as in tlie Brittle-stars. Now, the point to observe 

 is, that in correlation with this graduated difference 

 in the mobility of the rays, there is a correspond- 

 ingly graduated difference in the development of 

 the ambulacral system of suckers. For in Echinus 

 this system is seen in its most elaborate and efficient 

 form ; in the common Star-fish the suckers are still 

 the most important organs of locomotion, though 

 the muscularity of the rays has begun to tell upon the 

 development of the specially ambulacral system, 

 the suckers not being so long or so powerful as 

 they are in Echinus. Lastly, the Brittle-stars and 

 ComatuhB have altogether discarded the use of 

 their sucking feet in favour of the much more 

 efficient organs of locomotion supplied by their 

 muscular rays ; and, as a consequence, their feet 

 have dwindled into useless rudiments, while the 

 rays have become limb-like in their activity. 



There is only one other point in connection with 

 the natural movements of the Echinodermata which 

 it is necessary for me to touch upon. All the 

 species when turned upon their backs are able again 

 to right themselves ; but seeing, as I have just 

 observed, that tlie or!:»'ans of locomotion in the 

 different species are not the same, the methods to 

 which these species have to resort in executing the 

 righting manoeuvre are correspondingly diveise. 

 Thus, the Brittle-stars can easily perfoJ'm the need- 

 ful manoeuvre by wriggling some of their snake- 



