VORTICELLA 239 



beginning at the peristomial 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 ciliated ring being always coincident in 

 position with the thickening of the cuticle described in con- 

 nection with the contractile 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 mega- 

 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 



