THE OPELET. 167 



they had fallen, closely withdrawn into their shells. Sup- 

 posing them dead, they were taken out, when they directly 

 began to emerge ; and when returned to the jar with the 

 other Monodontas, they were all in less than five minutes 

 clustered round its mouth. On placing them again in the 

 jar with the Actinia, though kept there for two hours, they 

 did not once show themselves out of the shell. Once more 

 placing them along with the other shells, they exhibited 

 their former signs of life and activity. The experiment 

 was repeated several times with a large Littorina, with the 

 same result, evincing fear of the Actinia on the part of the 

 Mollusks."* 



I can only say that Trochus umbilicatus, Littorina littorea. 

 and Chiton fascicidatus have no such fear of Anthea cereus - 

 lor I have just seen these crawl without hesitation by the 

 side of a full-grown and vigorous specimen. 



Though sensible pain or irritation does not invariably 

 follow the contact of the human skin with the tentacles of 

 Anthea, yet their strong power of adhesion is never lacking. 

 Dissection reveals the cause of both, in the unwonted pro- 

 fusion with which these organs are furnished with cnidoz. 

 In the outer of the two layers of which the tentacle- 

 wall is composed reside the cnidw, excessively numerous 

 and thickly crowded ; of two kinds, chambered and sprraL 

 But it is in the crimson tips that the cnida? exist in the 

 most prodigious profusion. They completely fill the field 

 of the microscope, when a portion of the wall, flattened by 

 the compressorium, is under view, without the least space 

 free of them, not even a line or a point ; but overlying each 

 other like herrings in a barrel, yet maintaining a genera] 

 uniformity of direction. 



Within the cnidiferous layer, there is another of pig- 

 ment cells, visible to the naked eye as a dark brown or 



* Zoophytes, p. 12<\ 



