98 THE MICROSCOPE. July, 



The hydra is always accompanied by a parasite, or 

 rather an habitual guest belonging to the class of ciliat- 

 ed infusoria, and which, on account of the singularity of 

 its way of walking, has received the name of Trichodine 

 Louse. 



It is easy to observe and study it by taking one of the 

 polyx^s, placing it in a drop of water upon a glass slide 

 and submitting it to a magnifying power of about 250 

 times. One then sees moving about upon the whole 

 surface of its body and arms some little animalcule 

 whose general form reminds one very much of a quoit 

 or disc. These discs seem at first sight to touch each 

 other slightly on the surface of their host, to slide 

 lightly against him without touching him, and one asks 

 himself by what mysterious phenomenon — by means of 

 what organs — do they keep themselves erect and move 

 about with so much facility. A closer examination will 

 show. 



The Trichodine presents, as we have said, the form of 

 a quoit, or one of the surfaces of a quoit (fig. 1). That 

 one which is turned toward the covering of the hydra is 

 admirably formed for adhesion, the other is adapted to 

 obtaining nutrition. In looking at the upper surface, 

 while in a state of extension and immovability, this in- 

 fusoria presents to us a circle of hairs analogous to those 

 of the vorticella, but of which the direction is just the 

 opposite. This is rather a hairy tube (fig. 1) which is 

 used to conduct the food to a cavity upon one side of 

 the body (fig. 1, c). At the bottom of this cavity is the 

 mouth. The portion comprised in the circle, and which 

 constitutes the upper disc of the quoit, may, according 

 to the movements of the animalcule, either take a con- 

 cave form or, on the contrary, enlarge itself into the 

 form of a dome. 



If now we pass to the study of the lower or adhesive 



