128 G. H. PARKER. 



growth so rich as on iron plates covered with certain non-poisonous 

 paints. After six weeks' submergence such plates were often cov- 

 ered with a growth of animals two centimeters thick and so dense 

 that the surface of the plate could nowhere be seen. Thus of all 

 the metals tested, none were so favorable for growth as certain 

 painted surfaces. 



The animals that were found on the plates were five common 

 sessile species. Two of these were bryozoans, Bugula turrita 

 (Desor) and Membranipora pilosa (Linn.), two were tunicates, 

 Molgula inanhattensis (Dekay) and Botryllus schlosscri (Pallas), 

 and one was the common barnacle, Balanus eburneus Gould. It 

 is remarkable that the plates were without significant vegetable 

 growth. 



The plates exposed represented 36 different habitats : 6 on dif- 

 ferent single metals and 30 on the 15 possible couples. Of these 

 36 habitats, 30 were occupied by animals. 



Bugula was the most generally present. On all the plates where 

 animal growth occurred at all Bugula was found. The fact that 

 Bugula is a colonial animal and that the colony is attached to the 

 substrate by a very small stalk explains how it could grow from a 

 surface on which another animal in consequence of more intimate 

 relations to the poisonous substrate could not gain a footing. In 

 the 30 situations occupied by Bugula the colonies showed great 

 differences in size and in vigor. The largest colonies on plates 

 that had been in the water 53 days measured 2.3 cm. in height. 



Botryllus is the second animal in frequency of occurrence, hav- 

 ing been found 28 times in a possible 36. The largest colonies on 

 plates that had been 53 days in the water measured about 5 cm. in 

 diameter. 



Membranipora, the third animal, occurred 27 times in a possible 

 36. The flat colonies of this bryozoan were usually almost circular 

 in outline and the largest formed after 53 days' immersion meas- 

 ured 2.4 cm. in diameter. 



Molgula and Balanus each occurred 26 times in a possible 36. 

 After a submergence of 53 days the largest Molgula measured 

 about 1.5 cm. in diameter and the largest Balanus had a diameter 

 of about one centimeter and a height of about half that amount. 



The relations of the animals to the various metals can best be 



