On Pterodina valvata. 27 



depths into which it dives out of reach of any but low powers ; 

 and, moreover, it has a habit of forcing its dehcately flattened disk 

 between the interlacing filaments of the Algae, and of thus hiding 

 half its beauties from the observer. 



In order to study the animal with any chance of success, it 

 was obvious that I ought to find, not two or three specimens, but 

 two or three hundred ; as then, with a score or so in the same live 

 box, I should be sure to find some one or other resting occasionally 

 on or near the surface of the upper thin glass. This July I was 

 fortunate enough to find a spot where Pterodina swarmed in 

 countless thousands ; the water was alive with them. It was an 

 odd habitat. A large pond near Abbot's Leigh — the carp pond of 

 an ancient monastery — is a favourite hunting-ground of mine; it 

 hes in a natural hollow between fir plantations, and has been 

 formed by damming up the course of a streamlet which runs 

 through the hollow. Algae grow thickly on the stones of the old 

 weu', and Melicerta and Floscularia are generally to be found 

 there with many free-swimming rotifers. I had pretty nearly fiUed 

 all my bottles, when I thought I would take a dip in the square 

 hole outside the pond in which the iron shaft of the weir gate 

 works. It is not more than a foot square, and is as dark and 

 unpromising a place as one could imagine; but here I found 

 Pterodina imtina, and a new species, swarming in countless 

 numbers ; and I have since obtained similar swarms of Triarthra 

 longiseta from the same place. 



Now I found T. longiseta before swarming in an open farm- 

 yard pond, of which the water was so coloured with manure, that 

 it was impossible to see through an inch thickness of it ; and yet 

 here it throve in comparatively clean water, but in a dark hole to 

 which light had scarcely any access. Certainly the ways of rotifers 

 are puzzling. 



There was a little duck-weed in the hole, and I found, when I 

 brought my captives home, that Pterodina dehghts in fastening 

 itself by its sucking foot to the stem of this plant ; so having by a 

 hand lens made out a stem with a score or so of rotifers on it, I 

 suddenly whipped it out of the water, and placed the whole party 

 in the compressorium. By snipping ofi' the green top of the 

 weed, and curling the stem into a circle, I had a natural cage 

 which held them in a moderate compass. 



I do not think I ever beheld a more beautiful sight than that 

 which the §rds objective, illuminated by Eoss' Aths condenser and B 

 stop now gave me. From thirty to forty of these animated fairy 

 shields of ghttering glass were swimming in every direction across 

 the field, or adhering to the plant, so as to be seen from every 

 point of view ; while some had most considerately attached them- 

 selves to the glass cover, and were as quiet as rotifers ever are. 



