210 COMPARATIVE ANATOMY 



beginning at the vestibular end divides the body into two halves, 

 both of which are at first attached to the same stalk. But one 

 moiety develops an additional ring of cilia at its posterior end, 

 this cilated ring being always coincident in position with the 

 thickening of the cuticle described in connection with the con- 

 tractile fibrils. It then becomes detached from the stalk, swims 

 away by means of its ciliated band, .and after a time attaches itself 

 to some object by its posterior end, forms a new stalk for itself, 

 loses its posterior circlet of cilia, expands its peristomial field, 

 and so develops into a new Vorticella. The other product of 

 division remains attached to the stalk, and thus one may 

 speak of it as the parent form, whilst the detached free- 

 swimming individual may be called the child. No such 

 distinction was possible in the Protozoa described in the 

 preceding pages. 



Though our information on the subject is somewhat meagre 

 there is no doubt that Vorticella is as little able to propagate 

 itself indefinitely by binary division as Paramecium. Speci- 

 mens cultivated in an organic infusion will flourish and multiply 

 by division for three or four days, but will then dwindle and 

 disappear if conjugation does not take place. 



The conjugation of Vorticellids presents many special features 

 which must be described in detail. It is easy enough to obtain 

 specimens in the act of conjugating, but it is very difficult to 

 follow out all the steps, partly because of the attachment of 

 the creatures to foreign substances, partly because the macro- 

 nucleus breaks up at an early stage into a number of minute 

 fragments and thus obscures the micronuclei. The various 

 steps have been followed more completely in Vorticella moni- 

 lata than in any other species, and again we owe the most 

 complete and accurate account to Maupas. 



The gametes of Vorticella are of two kinds, macrogametes 

 and microgametes. The former are stalked individuals differ- 

 ing neither in size nor in any other obvious feature from normal 

 specimens. The microgametes, as their name implies, are 

 much smaller, and they differ further from normal individuals 

 in the possession of a posterior circlet of cilia. They have no 

 stalk, but swim freely through the water by means of this ac- 

 cessory ciliated band* 



In V. monilata the microgametes are formed by the binary 

 division of an ordinary stalked form. Sometimes the products 



