UEiMlODUCTION AND LIFE CYCLES Ml 



has been previously shown that in Hydra the new bud extends out 

 from the body, developing tentacles, mouth, and hy[)ostome at the 

 distal end of the organism. After growing sufficiently the base 

 constricts and the two animals, parent and offspring, become sepa- 

 rated, each taking up an independent existence (page 184). 



In the higher worms such as the palolo worm and the Naididae, a 

 type of budding occurs which might be described as fragmentation. 

 The number of fragments apparently depends upon tlu; size of the 

 worm, each piece usually producing all of the missing parts. 



Fission 



This variety of asexual reproduction is the most common. The 

 one-celled protozoa rely almost exclutijvely upon this type of develop- 

 ment, seldom resorting to the more complicated "sexual" methods. 

 In binary fission the nucleus appears to take the initiative, since it 

 divides first and is followed by the division of the cytoplasm of the 

 cell. 



Fission is rather closely allied to budding. Many of the turbel- 

 larian and nemertin(\an flatworms utilize this method, as, for example, 

 the turbellarian, Microsiomum, which often divides into two, four, 

 or even sixteen pieces. These parts produce all of the necessary 

 structures except eye-spots and often remain attached in chains for 

 long periods of time. 



Sexual Reproduction in the Invertebrates 



Protozoa 



Sexual reproduction involves the union of two cells produced usually 

 by two animals of different sexes. This phenomenon appears in 

 practically every group of the animal kingdom. Even in the protozoa 

 there are two types of reproduction which may be thought of as 

 initiating the sexual method. In the first type there is either a 

 complete union of two individual cells of equal or of unequal size, 

 or there may be specialized cells called gametes. Many variations 

 of this type are to be found among different species. 



The second type of sexual reproduction occurring in the protozoa 

 is called conjugation, which has already been described (page 161). 

 Briefly, conjugation means that two single-celled organisms come 

 together temporarily, form some sort of protoplasmic bridge, exchange 



