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THE ROTULA. 



the development of a circular disc which will represent the face of the clock, upon 

 which are traced certain lines that answer to the hands and figures. By rapid 

 degrees, the disk expands and covers the gelatinous substance of the animal, and puts 

 on hour by hour more of the Echinus as it loses its former skeleton shape. The latter 

 becomes rapidly covered by and absorbed into the former, and in due time the frame- 

 work of long slender rods, which might also be well compared to an artist's easel, or 

 the tripod stand of the theodolite, is converted into the well-known globular Echinus, 

 with all its complicated apparatus of spines, pedicillariae, and walking organs. 



The reader may perhaps have noticed, that on inspecting a common Echinus, 

 especially from the interior, it exhibits in a very distinct manner its close alliance with 



LOBED URCHIN. 



'F.ncope graadis. 



KEYHOLE-URCHIN. 



'Eacope subclausa. 



WHEEL URCHIN.- 



Rotula Rumphll. 



the well-known star-fishes. Take, for example, a common five-finger star-fish out of 

 the water, lay it on its back, and then gather all the five points together. Now, 

 supposing the creature to be dead, strip the skin from the rays, leaving it only adherent 

 down the centre, join the edges of the strips, and there is a very good imitation of the 

 Sea-urchin. 



THE two curious specimens represented in this illustration also belong to the Shield- 

 urchins. 



The ROTULA, so called because of its wheel-like shape, is nearly as flat as a piece 

 of money, and has a very slight elevation in the centre. It is remarkable for the very 

 deep teeth into which one side of the disc is cut, giving the creature an aspect as if it 

 were a cog-wheel in process of manufacture. The color of this species is mostly 

 grayish slate above, and dull white below. The under surface is veined over its whole 

 extent, all the veinings radiating from the centre. The color of this Rotula, is, how- 

 ever, extremely variable. 



