STAR-FISH AND SEA-URCHINS. Gis 
mobile rays. Midway in the 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 the 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 Kchinus. Lastly, the Brittle-stars and 
Comatulze 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-lke 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 the organs of locomotion in the 
different species are not the same, the methods to 
which these species have to resort in executing the 
righting manceuvre are correspondingly diverse. 
Thus, the Brittle-stars can easily perform the need- 
ful manceuvre by wriggling some of their snake- 
