HISTOLOGY OF SPONGES. 271 



a thing is accomplished it is no reason that we should ignore the 

 fact. How is it that Lucerne can penetrate its roots two or three 

 feet into the chalk, or the adventitious roots of the ivy penetrate 

 frequently through substances, even limestone, round which the 

 plant is growing ; one can hardly grasp the idea of the process 

 being mechanical or even chemical, when we consider the delicacy 

 of the soft vegetable cells composing their framework, particularly 

 those at the ends of the rootlets which are working their way in. 



I am not rash enough to say that it will never be found out how 

 Cltona accomplishes its excavations, but I do think that this family 

 of Sponges will continue to burrow into shells and rocks, and bore 

 naturalists for some long time to come, without the modus operandi 

 being thoroughly understood. 



As I have said before, I certainly agreed with Dr. Bowerbank 

 until I came across shells where the cavities ramified right and left 

 into such fine processes, and those cavities being filled with the 

 Sponge I could not at all make the burrowing of Annelids agree 

 with them. 



In conclusion, I may mention that we hear of a great many 

 of the borings being made by lithodomous Annelids and Molluscs, 

 but I cannot find any account of the said Annelids and Molluscs being 

 found in them, or by what species they are formed. Dr. Bower- 

 bank's analogy of the process pursued by the Annelids being ana- 

 logous to that of the garden worm swallowing the earth and the 

 marine Annelids swallowing the calcareous substances from the 

 borings they make in the limestone rocks, hardly holds good, after 

 what Mr. Gosse tells us, that all marine Annelids are carniverous. 

 It must be rather hard fare for them if it is so. 



If time and weather had permitted, I had intended to have gone 

 to the coast, and treated the shells with osmic acid, and taken sec- 

 tions ; this I still hope to do at some future time. 



