56 HOMES UXDER THE SEA. 



The Echinode"rmata, then, form a connected series of 

 links between the Polyps on the one side and the Worm 

 tribe on the other. We shall only be able to trace the 

 links backwards towards the Polyps, leaving those which 

 connect the Starfish with the Worms to another oppor- 

 tunity. 



The reader will find the corals described in another 

 chapter, and several figures illustrating the structure of 

 the Polyps that deposit the various corals and madre- 

 pores. These Polyps are composed of a stem and stalk, 

 surmounted by a crown of radiating tentacles like the 

 petals of a dahlia. 



Now, supposing we have the footstalk greatly elon- 

 gated, and that the earthy particles, instead of being 

 placed outside the animal, as with the corals and madre- 

 pores, are deposited within its tissues, we shall have 

 an approach to a skeleton. Even if the animal were to 

 die and decay, the earthy particles would still remain, 

 and would, under some circumstances, retain the general 

 form of the animal in which they were deposited. 



We should then have a stony footstalk surmounted by 

 a crown of stony tentacles. 



Let us now take another step. 



It is evident that such a creature would be unable to 

 live, as it would be without flexibility, and would be 

 snapped asunder by any movement of the water in which 

 it lived. But, supposing that the earthy particles were 

 deposited in a series of solid pieces, and each piece 

 connected with the others by the soft parts, we should 



