n8 ELEMENTARY ZOOLOGY 



special tactile or touch organs in all the Echinoderms, 

 and the starfishes have very simply composed eyes or 

 eye-like organs at the tips of the rays. 



While some of the Echinoderms breathe simply through 

 the outer body-wall, taking up by osmosis the air mixed 

 with the water, some of them have special, though very 

 simple, gill-like respiratory organs. These organs con- 

 sist of small membranous sacs which are either pushed 

 out from the body into the water, or lie in cavities in the 

 body to which the water has access. There is also a dis- 

 tinct circulatory system, but the " blood" which is 

 carried by these organs and which fills the body-cavity 

 consists mainly of sea-water, although containing a 

 number of amoeboid corpuscles containing a brown pig- 

 ment. There is no organ really corresponding to the 

 heart of the higher animals. There are distinct organs 

 for the production of the germ or reproductive cells. The 

 sexes are distinct (except in a few species), each individual 

 producing only sperm-cells or egg-cells, but the organs 

 or glands which produce the germ-cells are very much 

 alike in both sexes. There is no apparent difference 

 between male and female Eehinoderms except in the 

 character or rather in the product of the germ-cell pro- 

 ducing organs. A few species are exceptions, certain 

 starfishes showing a difference in color between males and 

 females. 



As all of the Echinoderms except some of the feather- 

 stars can move about, they have organs of locomotion, 

 and well-defined muscles for the movement of the loco- 

 motory organs. The external organs of locomotion, the 

 tube-feet (in the sea-urchins the dermal spines aid also in 

 locomotion), are parts of a peculiar system of organs 

 characteristic of the Echinoderms, called the ambulacral 

 or the water-vascular system. This system is composed 

 of a series of radial tubular vessels which rise from a cen- 



