The Marine Aquaritim. 173 



Already I have made out from it several species 

 that have been long nameless in my collection. 



"The apparent immutability of creatures so low 

 in the scale of being seems most wonderful. I find 

 that we have some of the same species here as are 

 found at Messina, in all their entirety of form, in- 

 dependent of the difference of influences in constant 

 operation between two places so widely apart. Por- 

 cellidittm fimbriatum is one of the species which I 

 refer to. It is a tenacious sucker. It often adheres 

 so firmly to the vessel in which it is placed that I 

 have to use a little spirits on the brush to get it re- 

 moved. But so far as I can make out, Dr. Claus 

 does not notice this fact. Perhaps he has not had 

 the opportunity of seeing the animal alive himself. 



"Mrs. Robertson and my two sons join me with 

 kind regards. Hoping that you all got safely home 

 and well, 



" Believe me, my dear sir, yours truly, 



"DAVID ROBERTSON." 



The friendship and correspondence with Professor 

 Koelliker was begun by the Professor's consulting 

 Mr. Robertson on the proper proportion of seaweed 

 to seawater required for the maintenance of healthy 

 conditions in an aquarium. 



The general notion at that time was that the water 

 in the aquarium could not be kept wholesome for the 

 marine animals unless some of the more or less bulky 

 algae could be induced to live in it with them. The 

 transplanted weeds frequently died, and in consequence 

 poisoned the water they were intended to purify. 



