CILIA OF MUSSEL. 63 



that free particles, blood-corpuscles, and other small 

 objects are moved along in it. But if the group is 

 small, or especially if entirely isolated cells are seen, 

 it will generally be found that the cilia act upon the 

 pieces to which they are attached like little paddles, 

 moving them about in the fluid. The cells, it may 

 be observed, are either shortly columnar or are 

 spheroidal ; the nucleus is seldom distinct, because 

 concealed by the granular nature of the cell-proto- 

 plasm. The cilia themselves can best be seen when 

 they are moving languidly or when their motion has 

 altogether stopped; they are very fine, and spring, a 

 number together, from the base or free surface of the 

 cell. 



Preparation 8. Cilia of Mussel. But by far 

 the most convenient object for the study of ciliary 

 motion is to be found in the gill of the common sea- 

 water mussel (Mi/stilus editus). Here the cilia are 

 very large, and their motion will go on unimpaired 

 for many hours. Hence they are particularly well 

 suited for the observation of the action of most of 

 the reagents which affect ciliary movement. 



One or more mussels may readily be procured at 

 any fishmonger's; those only should be chosen which 

 remain tightly closed, for those with open valves are 

 in most cases already dead. One of the shells may 

 then be forcibly separated by means of a knife, when 

 the gills (Fig. 16, br) will come into view, as flattened 

 expansions of a yellowish color, covering a consider- 

 able part of the shell, inside its lining membrane m 

 By observing carefully it may be noticed that they 

 have a striated aspect, the markings passing trans- 

 versely to their length, and by taking up a small 

 portion with a forceps it will further be seen that 

 this striation is due to the fact that the gill is made 

 up of a number of little bars which are distinct from 

 one another for the greater part of their length. 

 Take now a small piece of the gill, including three 

 or four of the bars, and placing it upon a side in a 

 drop of the sea- water which the shell always contains, 



