354 NATURAL HISTOEY. 



sorbing pores are all on the outside, while the vents are 

 inside. The result is that there is a strong current of 

 water constantly pouring out of the mouth of the bottle. 



Questions. How do the Polypes differ from the other classes of Ra- 

 diates in regard to locomotion ? Why were they so long supposed to 

 be plants ? What is said of their construction ? What is said of the 

 .structure of the Hydra ? What of its mode of taking its prey ? What 

 of its locomotion ? What is stated by Trembley ? Describe the two 

 ways in which Hydras are multiplied. What is said of mutilating 

 them ? What of uniting two together ? What is said of the Helianthoi- 

 da? Where are the Actiniae commonly found ? What is said of their 

 multiplication from portions of their disk ? What is said of their mus- 

 cular structure ? What of their mouths ? What of their voracity ? 

 What is the structure of their tentacles? What is said of the skele- 

 tons which some Polypes have ? How does their composition differ 

 from that of the skeletons of common animals ? In what other re- 

 spect do they differ ? How is the formation of the skeleton column 

 exemplified in the Caryophyllia ? Whence comes the supply of the 

 material to make this skeleton ? What is said of the associated Pol- 

 ypes as exemplified in the Astrea Viridis ? What is said of the form- 

 ation of the coral reefs and islands? What of the agency of the coral 

 animals in the forming ages of the earth ? What is said of the Aster- 

 oida ? What is said of the structure of the Sponges ? How many 

 species are there ? Describe the arrangement of the Bottle Sponge. 



CHAPTER XXXVI. 



CONCLUDING OBSERVATIONS. 



IT is my intention in this chapter to retouch some 

 points which have been treated of, and also to bring out 

 some others which may add to the interest of the gen- 

 eral subject. 



618. The pupil has observed, as he has proceeded, the 

 adaptation of each animal to its circumstances and to its 

 mode of life. This has been seen both in classes of ani- 

 mals and in individual cases. I will refer to a few ex- 

 amples of this adaptation in classes. Birds are fitted in 

 both their internal and external structure (as you saw in 



