SEA-URCHINS 221 
The Echinoidea present great differences in shape, being more 
or less spherical, oval, discoid, and heart-shaped. These varia- 
tions are associated with the differences of internal structure, 
the openings of the digestive tract being at the opposite poles in 
the spherical and oval forms, but excentric in the disk- and heart- 
shaped species. The sea-urchins are grouped in three orders in 
accordance with these variations. All are characterized by the 
absence of arms, by having the calcareous plates immovably 
united to form a firm test, and by the great development of the 
movable spines upon the plates. 
Sea-urchins are sometimes called sea-eggs, perhaps from their 
shape, but possibly from the edible quality of some species, which 
are eaten by the natives of the shore, who take them at the 
spawning season, when the egg-sacs are distended. They are gre- 
garious, and frequently are so crowded together as literally to pave 
the surface of rocks and the bottoms of tide-pools in sheltered 
places. The following is quoted from A. Agassiz: “Many of the 
Desmosticha along coasts exposed to the action of the waves live 
in cavities which they hollow out of the solid rock. This they 
do, not by means of any solvent, but by mere mechanical action. 
They chisel out with their teeth the solid rock by incessant turn- 
ing round and round, and keep their cave, where they are fre- 
quently prisoners for the rest of their existence, up to the size 
required by the growth of their test and spines, by constant 
gnawing. On the coast of California the common Strongylocen- 
trotus purpuratus occurs in this way. We find long tracts of 
shore, where this sea-urchin is common, completely honeycombed 
and pitted by cavities and depressions in which they seek shelter 
against the powerful surf continually beating against the rocks. 
The same species does not excavate in sheltered places, where the 
sea-urchins can find protection between the interstices of large 
fragments of rock or ledges more or less sheltered from the 
more direct action of the open sea.” 
Sea-urchins in cavities of granite rock, where the openings are 
too small for the animal to get out, are to be seen in thousands 
on the coast of France at Croisic, Lower Loire. Spines of large 
sea-urchins are usedas slate-pencils by the missionaries in the 
Pacific Islands. 
