THE STARFISH AND SOME ALLIES 229 



on the scale of the dissection drawings. A very delicate 

 circular blood vessel lies just below the ring- canal of the 

 water- vascular system and sends a branch into each arm 

 below the radial canal. The body cavity is filled with a 

 fluid similar to the blood, which is colorless. The system of 

 blood vessels is not complete. 



The body gets much of the oxygen it needs by way of the 

 water-vascular system. The remainder comes in through 

 the many short-branched gills, that cover the aboral sur- 

 face between the spines like the pile of a soft mat. The 

 gills are thin-walled tubes continuous with the body cavity 

 through openings between the plates of the skeleton. Thus 

 the fluid within the body cavity also fills the gills. In the 

 gill this body fluid gives up its carbon dioxide to the water 

 bathing the surface of the gills, while at the same time dis- 

 solved oxygen in the water passes through the delicate walls 

 of the gills into the body fluid. As mentioned on page 227, 

 the intestinal caeca also probably aid in respiration. Car- 

 bon dioxide passes out through the same organs which bring 

 in the oxygen. 



The diagram shows nine bulb-like organs on the outer 

 margin of the ring canal. These are called the Polian vesicles 

 (Fig. 121, p). The small glandular bodies, Tiedemann's vesi- 

 cles (Fig. 121, t), which join the Polian vesicles are thought 

 to have the function of producing amcebocytes, described 

 on page 243. The cells on escaping into the body cavity 

 consume the waste substance of metabolism and make their 

 way through the body wall, perishing on the outside. Aste- 

 rias has no definite organs of excretion, like kidneys or 

 nephridia. 



The Nervous System. If an observer takes a live starfish 

 and parts the tube feet in an arm so that the animal's skin is 

 exposed between the second and third of the four rows, he 

 may see the dead-white, radial nerve cord extending along 



