SEA-URCHINS 221 



The EcMnoidea 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 used as slate-pencils by the missionaries in the 

 Pacific Islands. 



