Coscinodiscv s asteromphalus. By E. M. Nelson. 



699 



B 



A membrane closing the pipe had been seen by me upwards of 

 twenty years ago, but its sieve-like nature was not recognized, 

 neither was it thought to be present in each pipe, but was regarded 

 as an anomaly. It is so diaphanous that its presence was only 

 perceived from an edge in a broken specimen. It is not improbable 

 that the office of all these sieve membranes in diatoms generally 

 is to guard the internal protoplasm from bacterial attack, and at 

 the same time to allow water to freely circulate. The bore of this 

 pipe measures, with antipoint correction, 14^00 m - = 1 ' 74 micra, 

 so the fineness of the holes in the sieve covering it can be imagined ; 

 it forms an interesting speculation as to what size of hole a diatom 

 considers it unnecessary to cover with a 

 sieve membrane. We have advanced 

 some way since diatoms were said to be 

 the fossil shells of Infusoria, and their im- 

 perfectly resolved structures were thought 

 to be the ridges and corrugations on those 

 shells. The biologists of those days were 

 so fully occupied with the outward shapes 

 of these newly discovered organisms that 

 the only thing they cared to know was 

 whether it was elongatum or attenuatum, 

 minutum or parvum, grande or major. As 

 all this quite unimportant detail could be 

 determined and surface markings, which 

 did not exist, be seen with Dr. Ehren- 

 berg's cheap and inefficient Microscope, 

 it was thought that a better Microscope, 

 or an instrument of the highest precision, 

 was quite unsuitable for biological pur- 

 poses. We now know that Nature's deep 

 secrets are not only beyond our unassisted 

 vision, but are also far beyond our most 

 powerful Microscopes, and the Microscope 

 necessary for biological work has not yet been and probably never 

 will be made, for there will always be a something beyond what 

 we can either see or understand. 



Diatoms are now known to be of great importance, and to 

 have a profound influence on biology, for it is diatoms which, 

 living on inorganic chemical substances in the sea, incorporate 

 those substances in their bodies, and change them into such a 

 form that they can be assimilated by the minute Crustacea and 

 Copepods that eat diatoms. The Crustacea are the food for higher 

 fishes, and so on ; the question now becomes one of economic 

 importance, for if we want to increase the number of edible fish, 

 we must increase the number of diatoms. What do we really 

 know about diatoms? Very little indeed; their life-history and 



3 A 2 



C 



Fig. 105. 



