OBSERVATIOXS ON FRESH-WATER POLYZOA. 107 



Besides these, there are o'onerally present a g'reater or smaller number 

 of cells or fragments of cells of a c|nite différent appearance, which 

 have prc)hably detached themselves from some part of the body. 



The floating elements were observed by previous investigators 

 (Allman, Hyatt, Verworn), but no great im])ortance was attributed 

 to what were brobably either parasitic organisms or detached cells. 

 Hyatt, for instance, observed "numerous organisms, many of which 

 probably parasitic, which float in the flnid, sometimes in such "a 

 number as to interfere with the examination of tlie internal structure." 

 It is prolmble that at least some of these " organisms '' were what I 

 regard as tiie blood corpuscles. 



The direction of the blood currents as (feerved in the natural state 

 is show^i in fig. 21. On the anal side of the body cavity the fluid is 

 driven toward the free end of the polypide, evidently by ciliary 

 action, whicli however could never be actually Ijrought to view^ In 

 the lophophoral arms, tlie corpuscles travel ;dong the floor to their 

 ends, and either returii directly along the ceiling, or enter the 

 tentacles, in which they ascend on the side nearer the tip of the 

 loph(^phore, and descend along the opposite side. In the cavity of 

 the epistome, the fluid streams along the ceiling to its tip and com- 

 ing back ah:)ng the floor of that (^rgan, either enters the epistome 

 again, or goes to the tip of the lophophoral arm along its lower side. 

 On the oral side of the polypide, the fluid is always seen flowing 

 do ^vn ward. 



Allman and Hyatt deny the presence of cilia on the external wall 

 of the alimentary canal, but Yerworn (saw them at the end of the 

 stomach in Gristatclla. My observations in living specimens of 

 Pectlnatella confirms tlie statements of tiie last author. 



Both Allman and Hyatt observed that the cœnœcia of Lophopiis, 

 Cn^fatella, and Phnuatdla vitrea, readily emptied themselves of 



