42 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



MODIOLARIA MARMORATA AND THE SURFACE FILM. 



BY HAROLD SELLERS COLTON. 



That small organisms act to the surface film of the water very- 

 much as they would react to a solid substratum is a matter of common 

 observation. Thus we see Hydra and various colonial protozoa hang- 

 ing from the surface film, and such mollusks as Lymnoea and Physa 

 crawling on its under side. Although the writer has never seen it 

 reported, yet it is not strange that lamellibranch mollusks should 

 react likewise. To be sure few lamellibranchs use their foot in a 

 manner comparable to the broad crawling organ of the gasteropod, 

 yet young ones oftentimes use their ciliated foot in a similar fashion. 

 However, one morning last 

 winter at Naples, the writer 

 observed an adult Modio- 

 laria crawling on the surface 

 film by means of its bvssus. 

 Being at once struck by 

 seeing so large an animal 

 suspended by such slender 

 threads attaching it to so 

 uncertain a substratum, he 

 at once made a sketch of it 

 (see figure ). 



The specimen in question 

 was found hanging by three 

 threads and was already 

 secreting a fourth. In other 

 words it was progressing 

 exactly as Mytilus would 

 have done on a solid sub- 

 stratum. Where the threads 

 were fast to the surface the 

 film was depressed. Al- 

 though Lymncea and Physa 

 expose a large area of foot to 



the film, this animal, not so big it is true, hangs from three or four 

 points. 



View of Modiolaria marmorata, looking down 

 upon it as it was suspended from the sur- 

 face film. The threads represent the byssus, 

 and the "V" at the end, the depression of 

 the surface film. 



