150 Papers from the Marine Biological Laboratory at Tortugas. 



themselves to the same new conditions and live under them together for 

 thousands of generations, still show as characteristic physiological differ- 

 ences as free-living forms, it is good evidence of the fundamental nature of 

 these differences. Is such the case? The present paper is a preliminary 

 consideration of this question. The work on which it is based was per- 

 formed at the Tortugas Laboratory of the Carnegie Institution of Wash- 

 ington during the summer of 1910. 



MATERIAL. 



The material used in these experiments consisted of four species of ciliate 

 protozoa obtained from the alimentary tract of the large black sea-urchin, 

 Diadema setosum, which can be procured in practically unlimited quantities 

 from the coral reefs about the Tortugas. This form is particularly favorable 

 as a source of material, not only on account of its abundance but on account 

 of the high percentage of infection shown by it with respect to the protozoa 

 in question. Out of over 100 adult Diademas examined during the progress 

 of the work, not a single one failed to show one or more kinds of protozoa in 

 the intestine, which often fairly swarmed with them. Doubtless the large 

 size of the intestine, its pouch-like recesses in which stagnation of the food 

 can occur, and the character of the food itself are factors favoring the pres- 

 ence of protozoan parasites or, perhaps more correctly, protozoan commen- 

 sals, since none of the forms in question seem to be injurious to the host or 

 to live at its expense, except in so far as they use up a small portion of its 

 practically unlimited supply of food. 



The forms studied were of four kinds. Since apparently none of them 

 have been previously described or named, they are for convenience desig- 

 nated in the account that follows by the letters. A, B, C, and D. Of the 

 four, B is the most regular in its occurrence, having been found in 100 per 

 cent of the adult Diademas examined. It is a large, heterotrichous form 

 allied to Metopus, with a hook-like anterior end and showing considerable 

 individual variation in appearance. 



The form next in abundance is C, which is found in about 75 per cent of 

 the cases. It is a medium-sized, holotrichous form, elongated in shape, with 

 the anterior end of the body transparent and longitudinally striated and 

 the remainder filled with refractive spherules. A nucleus is visible about the 

 middle of the body and no mouth opening can be detected. 



The next form in importance is the one designated as D, which is found 

 in 40 per cent of adult Diademas. It is flat and leaf-like in form, with the 

 body slightly concave on the ventral and convex on the dorsal surface. The 

 one margin of the body is almost straight and the other curved, giving the 

 animal an asymmetrical shape. At the posterior end is a spur-like pro- 

 jection. Evidently this form is allied to the holotrichous genus Dys terra. 



The form which occurs in the smallest number of cases (33 per cent) is 

 A. When present at all, however, the number of individuals is generally 

 very large. In size it is rather small and in shape elliptical and flattened. 



