BRANCH ECH1NODERMATA : STARFISHES, ETC. 119 



tral circular or ring vessel and which give off branches to 

 each of the tube-feet. The water from the outside enters 

 the ambulacral system through a special opening, the 

 madreporic opening, and flowing to the tube-feet helps 

 extend them. The tube-feet usually have a tiny sucking 

 disk at the tip, and by means of them the Echinoderm 

 can cling very firmly to rocks. 



Development and life-history. Differing from the 

 sponges and the polyps and jellyfishes, the reproduction 

 of the Echinoderms is always sexual ; young or new indi- 

 viduals are never produced by budding, or in any other 

 asexual way. The new individual is always developed 

 from an egg produced by a female and fertilized by the 

 sperm of a male. The eggs are usually red or yellow, 

 are very small (about ^ in. in diameter in certain 

 starfishes), and are fertilized by the sperm-cells of the 

 males after leaving the body of the female. That is, both 

 sperm-cells and unfertilized egg-cells are poured out into 

 the water by the adults, and the motile sperm-cells in 

 some way find and fertilize the egg-cells. 



From the egg there hatches a tiny larva which does 

 not at all resemble the parent starfish or sea-urchin. It 

 is an active free-swimming creature, more or less ellip- 

 soidal in shape and provided with cilia for swimming. 

 Soon its body changes form and assumes a very curious 

 shape with prominent projections. The larvae of the 

 various kinds of Echinoderms, as the starfishes, sea- 

 urchins, sea-cucumbers, etc., are of different characteristic 

 shapes. The naturalists who first discovered these odd 

 little animals did not associate them in their minds with 

 the very differently shaped starfishes and sea-urchins, but 

 believed them new kinds of fully developed marine 

 animals, and gave them names. Thus the larvae of the 

 starfishes were called Bipinnaria, the larvae of the sea- 

 urchins Pluteus, and so on. These names are still used 



