354 NATURAL HISTORY. 
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 Actiniz 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. 
Ir 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 
