12 MECHANICAL FUNCTIONS. 



little animal passes through any number of times without 

 injury. In like manner the gordius, a worm resembling 

 a horse-hair, which inhabits stagnant pools, may be dried, 

 when it has no more signs of life than a piece of wire, 

 and again revived to life by immersion in water. This 

 animal is supposed by many to have derived its exist- 

 ence from a horse-hair accidentally falling into the 

 water. But it is hardly necessary to say that such mis- 

 takes call for a more general knowledge of animal Phys- 

 iology. 



5. We shall begin our physiological descriptions with 

 the most simple organizations, and gradually passing 

 through those which are more and more complex, finally 

 come to that of our own species. 



SPONGE. 



6. The remains of this animal are in such universal use, 

 and consequently so well known, as to require no general 

 description. It belongs to an order of animals called 

 zoophytes, which also includes the corals, the polypi, 

 and several other races which are only a single grade 

 above vegetables. This order indeed appears to be the 

 connecting link between the animal and vegetable king- 

 doms. The term zoophytes signifies " animated plants." 



7. The sponges of which there are many species, are 

 all marine animals, living at the bottom of the sea, where 

 they are firmly attached to rocks and stones. 



8. These productions in general appearance much more 

 nearly resemble plants than animals ; but in their internal 

 organization and structure they diifer entirely from vegeta- 

 bles. Their animal nature is clearly shown by chemical 

 analysis, and by the voluntary motion of some of their 

 parts which resemble respiration. 



9. Every part of the surface of a living sponge presents 

 to the eye two kinds of orifices ; the larger having a 

 rounded shape, and generally a little raised on the mar- 



What is said of the gordius? To what order of animals does the sponge 

 belong? What is the meaning of the term zoophytes ? How is the animal 

 nature of the sponge indicated ? What is said concerning the orifices of 

 the living sponge ? 



