548 THE KEYHOLE-URCHIN. 



arrangement of the plates from which the shell is built may be seen in the figure. The word 

 "placenta" is Latin, signifying a flat cake, and is appropriately given to this species. 



The development of the Echinus is so very remarkable, that it deserves a passing notice. 

 This creature passes through a metamorphosis even more strange than that of the insect, and 

 no one who was not acquainted with the animal could possibly recognize in the delicate frame- 

 work of translucent spines the larval form of the globiilar Sea-urchin. At first, the little 



creatures are almost shapeless and globular, roll- 

 ing about through the water in an uncertain kind 

 of way. But by degrees they put forth a dome- 

 like portion, from which proceed several slender 

 calcareous rods, altogether making a figure that 

 has been aptly compared to a skeleton French 

 clock. In this state it was formerly known by the 

 name of Pluteus. As if to carry out the compari- 

 son still further, the first indication of change to 

 its more perfect form is the development of a cir- 

 cular disc which will represent the face of the 

 clock, upon which are traced certain lines that 

 answer to the hands and figures. By rapid de- 

 grees, the disc 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 

 (Natural size.) shape. The latter becomes rapidly covered by 

 and absorbed into the former, and in due time the 



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

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

 its complicated apparatus of spines, pedicillarise, 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 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 Cake-urchin is represented on the New England coast by one of about three inches 

 diameter. This object is, perhaps, more puzzling to the average observer than any other. Its 

 remarkable flatness is a stumbling-block to understanding it as an animal. Sand Cake is a 

 name given it, and suggests its possible origin with the uninformed. 



Two other curious members of this genus are the KEYHOLE-URCHIN and the WHEEL-URCHIN. 

 The latter, 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 species is, however, extremely variable. It is also called 

 Rotula. 



The second species might be well called the Keyhole-urchin. This remarkable creature, 

 instead of being toothed at the edge like the preceding species, has its disc pierced with 

 oblong apertures of a shape much resembling a keyhole. These apertures are rather variable 

 in their shape, sometimes being merely pierced through the disc of the Urchin, and sometimes 

 extending fairly to the edge. When full-grown, this is rather a large species, much resem- 

 bling an ordinary pancake both in shape and dimensions. There are many species of Encope, 

 most of which are inhabitants of the hotter seas, some being found in Southern America. The 

 color of the Keyhole-urchin is dull gray. The whole family is a very remarkable one, and 

 affords numerous points of interest to the careful observer. 



