1898.] NATURAL SCIENCES OF PHILADELPHIA. 113 



Little attention having been paid to the slower movements of 

 forms outside the Raphidiea3, it is not unusual to meet with state- 

 ments implying that definite and positive motion, requiring expla- 

 nation, is not observable in the case of these forms. This is a curi- 

 ous error. Splendid exhibitions of motile activity have long been 

 familiar in the case of various species of Nitzschia, and the exceed- 

 ingly striking movements of BaciUaria paradoxa are known to ob- 

 servers the world over. Certain species of Diatom a and Fragilaria 

 exhibit motile phenomena of no doubtful kind ; and of Eunotia 

 major, which may stand as a type of the Fragilariese, the follow- 

 ing are the observed facts : — 



When a drop of water containing errant frustules is observed 

 under an enlargement of about 200 diameters, the first movement 

 detected will probably be that illustrated in fig. 2, PI. VI. Certain 

 frustules rise slowly on their ends, rotating smoothly through 180°, 

 and fall flat upon the slide. During this manoeuvre an end remains 

 in contact with the glass and serves as a hinge whereon the frustule 

 turns. Occasionally, but not often, a similar revolution is ac- 

 complished at right-angles to this — in which case the long valve, 

 instead of the narrow end, functions as a hinge. This particular 

 movement only occurs immediately after the deposition of the di- 

 atoms upon the slide, unless at this time the cells are not in a state 

 of activity, in which case it may be somewhat delayed. A long 

 series of observations, extending over three years, has shown both 

 the conditions under which this odd movement occurs, and the 

 manifold purpose of it. It is, first, significant that only those cells 

 that have chanced to fall upon the slide with ventral or concave 

 girdle uppermost, as in A, fig. 2, Plate VI perform this movement. 

 Such as have been deposited with ventral side down, as in C, begin 

 without delay to move endwise in the direction indicated in the figure. 

 Careful focussing shows that in most cases the frustule is, during 

 this forward movement, tilted a trifle, so that while the anterior end 

 remains in contact with the glass, the posterior end is free from it. 

 This position is undoubtedly a condition precedent to successful 

 locomotion, and there is no doubt that the attaining of this position 

 is the purpose of the rotation. That the anterior end is in contact 

 with the slide will be proven conclusively as we proceed ; and it is, 

 therefore, to the end of the frustule, and more especially to the 

 ventral side of the rounded end, that search must be directed for 

 the mechanism of this diatom's movement. 



