1898] THE MOVE ME XT OE DIATOMS 409 



they remain separate give rise to the canals. Consequently, each of 

 these intermediate pieces forms a niche-like depression between two 

 projecting transverse canals and a corresponding piece of the 

 longitudinal canal. Lobular processes of the chromatophores pro- 

 ject into the transverse canals and are always enveloped by proto- 

 plasm, which passes from their extremities towards the outer wall 

 as a strand, usually undivided. These strands exhibit both longi- 

 tudinal and transverse fibrillation during life, and, as a rule, appear 

 to consist of five or six longitudinal rows of cellular compartments. 



The longitudinal canal was described by Flogel as closed exter- 

 nally, but Lauterborn's sections show that it is interrupted along its 

 outer edge by a very narrow cleft, thus placing the interior of the 

 cell in direct communication with the surrounding medium (Fig. 4>). 

 This cleft is not only visible in sections through the frustule, but 

 can also be made out in a surface view when one of the alae is 

 viewed vertically, parallel to the direction of the transverse canals. 

 Yet another circumstance, not taken into account by Flogel, points 

 to a breach of continuity in the cell-wall at this spot. In the living 

 Surrirella, foreign bodies (e.g. particles of Indian ink, sand, small 

 diatoms,) are readily observable adhering along the edges of the 

 alae, where they are moved briskly to and fro in the same manner as 

 along the raphe of Einnularia, Navicula, etc., a circumstance very 

 difficult to explain if the cell-wall were in reality unbroken. 



Most of the Surrirella were infested externally by a small 

 alga belonging to the Cyanophyceae (Chroococcus sp.). The small 

 blue-green spherical cells always occurred in a definite situa- 

 tion, viz., in the niche-like intermediate pieces of the alae, only a 

 single alga being present, as a rule, in each cavity (Fig. 5 pa.). 

 More than once, examples of SurrirelUt were noticed where all the 

 intermediate pieces of the alae sheltered these little lodgers. It is 

 pointed out that the advantage may well lie exclusively on the side 

 of the Chroococcus, a possible explanation being that the alga, seated 

 on the diatom in the very fine and easily disturbed mud, suffers less 

 interruption of the assimilative processes than if it were free in the 

 mud, because the diatoms, if buried by the movements of fishes or 

 creeping molluscs, soon work their way up again to the surface 

 and so to the light. In this connection it may be mentioned that 

 Gastrotricha, of the genera Chactonotus and Ichthydiinn, constantly 

 attach their eggs to the surface of large Surrirellae. 



The Movement of Diatoms. — Various hypotheses have been 

 advanced to account for the characteristic movements more or less 

 familiar to every student of the Diatomaceae. Amongst earlier in- 

 vestigators, M. Schultze held that protoplasm, protruded through the 

 slit-like raphe, served to set the cell in motion, and this view was 

 maintained by Pfitzer and Engelmann. On the other hand Nageli, 



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