CHAPTER XIX 

 HYDRA: A STUDY OF ATTACHED ANIMALS 



A COMMON distinction hotwecn plants and animals is that 

 plants are attached and rooted to the soil, whereas animals are 

 able to move from place to place. Although this distinction 

 holds in the case of most animals and plants it is not without 

 numerous exceptions. Tlius there are many lowlj' organisms 

 which are assigned to the plant kingdom that have never lost 

 the power of s^vimming about. This is true of the small, green 

 organisms that live on the surface of the sea or in the waters of 

 ponds. It is also true of the whole group of liacteria that play 

 so important a part in disease. On the other hand, there are 

 a few animals that Iiave lost the cajiacity for locomotion in 

 their adult stage and are rooted to the substratum like plants. 

 There are four different t3'pes of attached or sessile animals : 

 (1) the Sponges and (2) the group of C'adenterata, both of 

 which we shall consider in the present chapter ; (3) the ]Moss- 

 animals or Bryozoa considered in Chapter XII; and (4) the 

 Mantle-animals or Tunicates considered in Chapter XXI. In 

 their young stages all of these animals are free-s\^imming, 

 but throughout their adult life are attaclicd to the solid objects 

 of the Ijottom. We are lioiuid to conclude^ that all attached 

 animals have Ijeen derived from free-swimming ancestors, 

 and we have therefore two questions to answer: "Why should 

 any free-swimming form evolve into an attached one, and Wiat 

 changes have attached animals undergone in abandoning 

 their free life? 



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