1890.] Excavations by Sea-Urehins. 19 
The almost universal presence of a calcareous alga on the rim 
of the sea-urchin boring might seem to indicate that there is a 
symbiosis between the alga and the animal. It would seem as if 
the rim was built up, in part at least, by the alga, as a protection 
for the Sea-urchin, and that in return the alga received certain 
advantages. In the case of very deep sea-urchin excavations 
this would not hold, as there. can be no doubt that the cavity 
is made by the urchin boring in the rock. 
Among other statements which might well be quoted, Caillaud 
reaches the following conclusion, which is very significant 
Speaking of rock excavations, and of the buccal armature, he 
asks, “ S'il n’était pas dans la nature de ces êtres de creuser des 
roches, pourquoi seraient-ils ainsi pourvus et outillés de poingons 
en émail dont les sommités s’usent, puis se renouvellent, se cra- 
courcissent dès lors et doivent, de toute nécessité, recevoir le pro- 
longement nécessaire pour conserver leur longueur voulue tant que 
Voursin travaille? Si cet appareil,” he continues, “si bien com- 
biné pour agir sur la pierre n’avait pas ce but, il deviendrait inutile 
a cette tribu de veritables Echinus que en sont pourvus ; ils auraient 
reçu des simples dents fixes et ordinaires, comme tant d’autres 
dans cette immense famille, comptant, comme nous l'avons dit, en 
vivants et fossiles, seize cents espéces. Comme example, nous 
citerons particulièrement le genre Clypéaster, à qui tous les 
moyens de perforer les roches sont interdits.” 
It is interesting, taken in connection with the above quotation, 
to speculate as to the character and use of the “Aristotle’s Lan- 
tern” in the Clypeastroids, and its absence in the Spatangoids. 
The lantern of the genus Echinarachnius is so small and incon- 
spicuous that it is difficult to believe that its function is the same 
as in the round sea-urchins, or echinoids. Moreover the manner 
of life of this genus is such that we can sil suppose that there 



31 Deshayes i pt the view th tth the rock. He regards 
them imply ki g out lready existing. He finds first t that it is without 
Isewhere in the an rid that th ies on one coast bores holes, 
and on another does not. Sesame: the habit once acquired would be universal for in- 
dividuals of the same species. Thirdly, th ve no organ for boring, and, 
lastly, the holes are cov with calcareous views have been discussed by 

others. a think, = answer to më nig and second sae sia we may say = tose wave 
i p bore, and in another do n 
D 

