CLASS SCOLECIDA. 21 



of its parts. The skin is more or less liardened or rough- 

 ened by means of lime deposited in it. There is a pecu- 

 liar system of tubes or "water-vessels," which contain 

 water, usually communicate with the exterior, and are 

 generally employed in locomotion. There is a distinct 

 nervous system consisting of a ring-like cord surrounding 

 the mouth, and sending off branches in a radiating 

 manner. These characters distinguish the class of the 

 Echinodermata as a whole. 



CHAPTER YII. 



CLASS SCOLECIDA. 



The class Scolecida (Greek, skolex, a worm) includes chiefly 

 the various worm-like animals which live parasitically in 

 the interior of other animals. Besides these there is a 

 niunber of nearly related forms, which lead a free exist- 

 ence, together with the singular group of the ^\^leel- 

 animalcules. One of these last we shall select as an 

 example of this class, not as being by any means a typical 

 example, but as not presenting certain disadvantages 

 under which other more characteristic forms labour. 



If we take one of the free-Living AVheel-animalcules, 

 such as Eosphora (fig. 8), we find that we have to deal 

 with a microscopic, translucent, httle creature, which is 

 to be detected swimming about actively in fresh water. 

 The name of " Wheel-animalcide " refers to one of its 

 most prominent peculiarities — namely, that the front end 

 of the body is in the form of a disc surrounded by a 

 fringe of little vibrating hair-like processes ("cilia"). 

 When these latter are in active motion, the whole head 

 looks as if it were rotating rapidly, like a wheel. By 

 means of this disc the animal not only drives itself 

 through the water, but also sets up currents which bring 



