No. 575] RESPONSES OF PLANTS AND ANIMALS 653 



(d) grand period of growth and the length of period of 

 internodes. 



The principles are concerned with asexual reproduction 

 and apply to motile organisms only exceptionally as for 

 example in the case of colonial pelagic forms. The laws 

 are applicable to both plants and animals. 



ii. Movements. — Movements of sessile animals are 

 usually contractions or extensions of parts or of the 

 entire body. Tentacles and comparable organs are capa- 

 ble of movements for securing prey. Such organs often 

 tend to wrap about objects which are in motion. Many 

 sessile animals are capable of opening and closing a 

 mouth opening and of bending or twisting the entire body. 

 Plants possess a comparable capacity only occasionally. 



(e) Behavior of Sessile Motile Organisms 

 Most sessile animals are capable of some movement 

 and react by contraction of parts. The reactions may be 

 modified by repeated stimulation (Jennings, '06) and 

 usually by physical factors. Some animals, as Hydra, 

 Stentor and many others are both sessile and vagile or 

 free-swimming, and show different types of behavior 

 when attached and when free. Jennings states that such 

 protozoa have a more complex behavior than motile 

 forms. This is due to their combining the types of 

 behavior of sessile and motile animals. 



(/) Response and Taxonomy of Sessile Organisms 

 Hickson ('98) has stated that there is but one species 

 of M ill c pore and believes that sex organs will be found 

 to be the best taxonomic characters. Wood-Jones states 

 that there are far fewer species of corals than has 

 formerly been supposed, and states further that growth 

 form can not be used to distinguish species. Among 

 fresh-water sponges and Bryozoa reproductive bodies 

 {gemmules and statoblasts) have been found to possess 

 satisfactory taxonomic characters. This is a situation 

 quite parallel with that in plants where reproductive 



