284 LEISURE-TIME STUDIES. 



to which we may, in zoological language, apply the term 

 41 polypite." This polypite is the most characteristic part of 

 the medusa in the eyes of systematic naturalists. At its 

 free extremity the mouth is found, and this aperture leads 

 into a hollow body-cavity, which is in its turn continued into 

 the " canals " that radiate through the body of the jelly-fish 

 and that are united by a circular vessel which runs round 

 the margin of the bell. Around the margin of the body we 

 also find tentacles or organs of touch, many or few, as the 

 case may be. In addition, we may observe certain struc- 

 tures known as " marginal bodies," which appear in the form 

 of spots of pigment named ocelli, these being rudimentary 

 eyes. And we also find at the rim of the bell certain little 

 sacs or bags containing limy particles suspended in a clear 

 fluid, these latter representing the rudiments and begin- 

 nings of organs of hearing. Thus the jelly-fish may be 

 found to possess a higher degree of organisation than might 

 at first sight be supposed. A closer examination of the 

 swimming-bell, which constitutes the bulk of the body, will 

 reveal the mechanism of its movements. The " polypite " 

 or stalked mouth, and the inner or concave surface of the 

 swimming-bell, are covered with a tissue which differs from 

 that comprising the body as a whole, in that it is highly 

 contractile. This contractile tissue may in fact be regarded 

 as representing the beginnings of muscle in the animal world ; 

 and through its agency the medusa is able to move grace- 

 fully through the yielding waters. When the layer of tissue 

 just mentioned contracts, the walls of the bell are pulled 

 together. The water contained within the cavity of the 

 bell is thus forcibly expelled, and by its reaction on the 

 surrounding fluid propels the jelly-fish onwards. The sub- 

 sequent relaxation and distension of the contractile layer 

 and swimming-bell permit a fresh inflow of water, pre- 

 paratory to the next contraction and succeeding expulsion 

 of fluid. One observation regarding the sensitiveness of 

 the medusa is worthy of remark, and that is, the special 

 localisation of its irritability and the regulation of its move- 



