

Aquarian Diatoms 



CHARLES 5. BOTER, A. M., F. R. M. S. 



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The moderate temperature of aquaria 

 facilitates the growth of diatoms through- 

 out the year. The thin yellow film, be- 

 coming darker when dry, clinging to the 

 glass sides below the surface of the water 

 and gradually creeping above the sur- 

 face on the parts more exposed to the 

 light, will be found, on examination by 

 the microscope, to be composed entirely 

 of the frustles of small diatoms. Al- 

 though the deposit is frequently removed 

 as an obstruction to the view, it must 

 not be considered detrimental to the life 

 of the aquatic fauna. On the contrary 

 the mucus surrounding the diatoms is a 

 source of food not only to the snails 

 employed in cleaning the walls but also 

 to the fish and the continued growth of 

 the frustules is an indication of the puri- 

 fication of the water as they are constant- 

 ly throwing off bubbles of oxygen. 



It is well known that the size of aquatic 

 animals is, to a certain degree, deter- 

 mined by the extent of the habitat and 

 that some snails which live in small ponds 

 do not essentially differ, except in size, 

 from larger forms occurring in lakes. 

 It will be found, therefore, that all dia- 

 toms in aquaria are of the most minute 

 dimensions, except, possibly, a few larger 

 forms which may occasionally appear in 

 the water supply but which do not persist 

 in growth. There are several diatoms 

 which, in this neighborhood at least, are 

 of constant occurrence, growing on the 

 walls of aquaria at all seasons. The fol- 

 lowing list is the result of numerous col- 

 lections made at different times in several 

 aquaria. 



Navicula minima. This form while 

 not quite the smallest as its name would 

 imply is only about two-thousandth of an 

 inch in length. It is linear in outline and 

 rounded at the ends, with fine lines radi- 

 ating from a longitudinal line in the mid- 

 dle, and may be recognized by the bright, 

 somewhat quadrangular space in the cen- 

 tre. The zone view is oblong. (Fig- I.) 



Bunotia pectinalis minor. The smaller 

 Comb Eunotia, so-called because it looks 

 like a comb with a curved back, being 

 convex on one side and concave on the 

 other, with quite visible transverse lines. 

 Its length is at least twice that of the 

 Navicula although the ordinary form of 

 Bunotia pectinalis is often five or six 

 times longer. When seen in another view 

 it is, like most diatoms, quadrangular in 

 outline. (Fig. 2.) 



Achnanthes exigua. This little form 

 always occurs, frequently in great abund- 

 ance, in any gathering made from aquaria 

 and has been kept living in a small two 

 ounce bottle for several years. The two 

 valves are unlike except in outline which 

 is rhombic-elliptical or sometimes quad- 

 rate with beaked ends. One valve is 

 crossed transversely by a blank line while 

 the other valve has only the usual longi- 

 tudinal line in the middle. The length is 

 about the same as that of Navicula mini- 

 ma. A quadrate form, constricted in 

 the middle, is frequently found with the 

 others. (Fig. 3.) 



Achnanthes linearis curta. This form 

 is elliptical, with a distinct longitudinal 

 line and a small blank space in the centre 

 in one valve and merely an indistinct 



