166 PRACTICAL ZOOLOGY. 



through the water. A set of needle-like rods develop 

 within, which soon make a skeleton shaped somewhat like 

 a common chair. This skeleton has a covering of soft 

 tissue, and the projections which correspond to the legs 

 of the chair are covered with strong cilia for locomotion. 

 The digestive tube has at first but one opening, that made 

 by the doubling-in of the outer wall, as above mentioned, 

 and the cavity of this depression forms the digestive 

 cavity. The mouth is formed later by a new opening 

 made through the outer wall into the first cavity and the 

 original opening becomes the anus. So far the young sea- 

 urchin is very unlike the adult; but after a time this larva 

 begins to transform into the real sea-urchin, and soon the 

 little sea-urchins, about the size of pins' heads, are found 

 crawling up the sides of the glass vessels in which they 

 are kept. 



The first of the changes here described should be care- 

 fully remembered, as this division, or segmentation, of 

 the egg is common to all but the very lowest animals, 

 though the manner of division may greatly vary. 



THE FRESH-WATER HYDRA. 



The fresh-water hydra has a cylindrical body, varying in 

 diameter from the size of a fine needle to that of a common 

 pin, and from one-fourth to one-half an inch in length. It 

 is found in fresh-water ponds and streams, usually attached 

 by one end to submerged stems, leaves, etc., frequently 

 on the under surface of a leaf. Surrounding the free end 

 of the hydra is a circle of thread-like appendages, the 

 tentacles, which often are longer than the body itself. 



