REPOET ON THE DIATOMACE^E. 5 



which encloses a greater or less number of regularly or irregularly disposed diatomaceous 

 frustules, was designated by Brebisson by the name of " Coleoderma." It is to be noted, 

 moreover, that solitary and free Diatoms are not completely devoid of external 

 mucus, but, on the contrary, are surrounded by a thin stratum of this substance. This 

 becomes manifest when their movements in water coloured with indigo are observed 

 under the microscope, inasmuch as the granules of coloured matter do not come 

 into immediate contact with the Diatom, but leave a thin translucent area surrounding 

 the frustule. 



But the most singular and curious phenomenon observable in some species, and one 

 which misled their first observers so far that they considered these organisms to be of an 

 animal nature, is the rectilinear motion with which, among others, the Naviculw, the Cym- 

 J>cUcB, &c, are seen to advance in the direction of their long axis and to return along the 

 same line. The strongest magnifying power, the most accurate and careful application of 

 the most perfect microscope, and the most skilful methods of observation, have failed to 

 discover the existence of any special organs of locomotion. This being so, the most 

 probable and generally accepted hypothesis suggested in explanation of the movement is 

 that the Diatom, in order to obtain the silex which constitutes its walls, must continually 

 absorb and reject the water which has in solution imperceptible traces of the siliceous 

 substance, and that the consequent action and reaction thus exerted upon the li^ht 

 frustule suspended in the watery medium, determines its alternate forward and backward 

 movement. 



Besides this phenomenon of motion another circumstance connected with these interest- 

 ing organisms early attracted the attention of naturalists, namely, the extreme delicacy 

 of the details with which their valves are ornamented. There is probably not a single 

 Diatom which, when examined by the best microscopes, does not show the surface of 

 its valves to be adorned by exceedingly minute granules, generally arranged in lines 

 which are usually spoken of as "striae," although properly they are lines or rows of 

 points. 



That these stria?, however, are sometimes perfectly continuous cannot be denied, although 

 some naturalists assert that the appearance of a continuous streak is purely illusory, 

 the so-called streak being but the expression of confluent granules. In this connection it 

 is to be noted that the phenomenon of diffraction produced by the obliquity and intensity 

 of the illumination has occasioned a belief in the confluence of the granules of pinnuloe, 

 &c, for example in Pinnidaria major, Raben., as such a result can be obtained at pleasure 

 in that and other frustules. This consideration, moreover, has led some to deny that there 

 is any value in the division of the Naviculaceos made by Ehrenberg when he instituted 

 the genus Pinnularia for navicular forms having pinnulae or coarse continuous strke, and 

 limited the idea of Navicula exclusively to navicular forms adorned by rows of granules. 

 Like the quality and delicacy of the ornamental details, the forms of the valves of 



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