302 NATURAL HISTORY. 
the stone uncovered by the 
flesh. Among the expand- 
ed Polypes are seen twe 
which are out of their cells, 
but their tentacles are not 
expanded. 
614. It is chiefly by the 
coral-forming animals of this 
order that the coral reefs 
and islands have been built. 
“s So immense are the works 
Fig. 273.—Mass of Astrea Viridis. | which large companies of 
these animals perform here and there, that we may re- 
gard the changes which they produce as among the most 
important to which the earth has been subjected, at least 
since it has been inhabited by man. <A Jarge number of 
the Polynesian Islands, and many of those in the Indian 
Ocean, have been constructed by these little animals. 
They are continually building extensive reefs, also, in va- 
rious forms and in different positions. Off the coast of 
New Holland there is a coral reef over one thousand 
miles in length. Great as are the changes now going on 
from the agency of these little architects, it is supposed 
that in what may be called the forming ages of our earth 
they had a still greater agency, in the formation of the 
limestone rocks which constitute so large a part of the 
crust of the globe. 
615. There is another order of Polypes called the As- 
teroida, from the star-like appearance of the tentacles. 
The Red Corals, the “ Organ-pipe Corals,” the Sea Fans, 
etc., belong to this order. Some in this order verge to- 
ward the sponges. Their habits are, for the most part, 
so much like those of the other Polypes that I will not 
dwell on them. 
616. The proper place of the Sponges it is difficult to 
determine. If they are really animals, they are of the 
lowest grade, exhibiting not the least signs of sensation. 
