548 
THE KEYHOLE- URCHIN. 
arrangement of the plates from which the shell is bnilt may be seen in the figure. The word 
“placenta” is Latin, signifying a fiat 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 globular 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 
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, 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 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. How, 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 Hew 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-ubchix and the Wheel- itechust. 
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. 
SHIELD-URCHIN . — Echinarachnius parma. (Natural size.) 
