38 A COMPOUND SPECIES 



but the result of such an accident taking place is 

 that the cilia cause the animal to swim through the 

 water, trailing its thread behind it, and the contrac- 

 tion of the latter merely causes it to be drawn up to 

 the head. 



There are various species of Vorticellse. That just 

 described is the simplest, consisting merely of a 

 hemispherical ciliated cup, attached to a single thread. 

 It is barely visible to the naked eye. But there is 

 a compound species which I have this year found to 

 be extremely abundant in my aquarium, whose 

 occupants, both large and small, it excels in singu- 

 larity and beauty. In structure it is to the simple 

 Yorticella what a many-branched zoophyte is to 

 an Actinia. My attention was first drawn to 

 the presence of this creature by observing some 

 pebbles and fronds of green ulva thickly coated 

 with a fine flocculent down. On closer inspection 

 this growth appeared to consist of a multitude of 

 feathery plumes, about one-sixteenth of an inch in 

 height, and individually of so fine and transparent a 

 texture as to be scarcely discernible to the unassisted 

 sight. On touching one - with the point of a fine 

 needle it would instantly shrink up into a small but 

 dense mass, like a ball of white cotton scarcely so 

 large as a fine grain of sand. In a few seconds it 

 would again unfold and spread itself out to its origi- 

 nal size. By carefully detaching a specimen with 

 the point of a needle or pen-knife, and transferring 



