160 MEMORANDA. 



in contact with, and quietly thrust aside heavy obstacles, 

 directly in their way, with a slow but decided mechanical 

 power, apparently only to be obtained from an abutment 

 against a solid body. In studying the motions of the Dia- 

 tomacece, 1 have frequently seen one get into a position such 

 as to become either supported or jammed endways between 

 two obstacles. In this case particles in contact with the sides 

 are carried up and down from the extreme ends, with a 

 jerking movement, and a strange tendency to adherence ; the 

 Diatom seeming unwilling to part with the captured particle. 

 Under these circumstances I have distinctly perceived the 

 undulating movement of an exterior membrane ; whether this 

 envelopes the whole surface of the silicious valves 1 am not 

 able to determine, nor do I know if the existence of such a 

 membrane has yet been recognized. The movement that I 

 refer to occupied the place at the junction of the two valves, 

 and is caused by the undulation of what is known as the 

 " connecting membrane." This will account for the pro- 

 gressive motion of the DiatomacecB, which is performed in a 

 manner analogous to that of the Gasteropoda. The primary 

 cause, however, is different, and not due to any property of 

 animal vitality, but arises, in my opinion, merely from the 

 effects of vegetable circulation. I have observed several cor- 

 puscles of uniform size, travel to and fro apparently within the 

 membrane, which is thus raised in waves by their passage. 

 From this I, therefoie, hazard a conjecture, with respect to 

 the movements of the DesmidiecB. (Their pi'ogression is but 

 seldom seen, and then extremely slow, and chiefly confined to 

 elongated specimens, as the Closteria.) As there are no indi- 

 cations of either external orifices or cilia, may not their loco- 

 motion be effected by the currents of protoplasm forcing their 

 way between the primordial utricle and outer tunic, which 

 will thus be raised in progressive waves, if the investment 

 happens to be in a suitably elastic condition ? — F. H, Wenham. 



Remarks on Jtlv. Wenham'a paper, on " Aperture of Object-glasses." 



— As Mr. Wenham now frankly admits the correctness of my 

 s|*itements with regard to the possibility of resolving difficult 

 test objects, even when balsam-mounted, no further remarks 

 are necessary upon that point, but a few words of comment 

 are required by other portions of Mr. Wenham's paper. 



That my reply was written before I could have had any 

 knowledge that Mr. Wenham had recalled his remarks, in 

 which doubt appeared to be thrown on my positive statement 

 of facts, will sufficiently appear by the date of my reply, which 

 was published in the 'American Journal of Science' for 



