THE STARFISH AND SOME ALLIES 243 



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 conies in through the 

 many short-branched gills, that cover the aboral surface be- 

 tween the spines like the pile of a soft mat. The gills open 

 into the body-cavity at their bases. Carbon dioxide passes 

 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. 120, p). The small glandular bodies, Tiedemanns vesi- 

 cles (Fig. 120, ), which join the Polian vesicles are thought 

 to have the function of producing amoebocyte-cells, described 

 on page 207. 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. Asterias 

 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 just 

 beneath the skin (Fig. 121, 17). Fig. 120 shows the position 

 of the nerve-ring just beneath the ring-canal. The circular 

 blood-vessel lies between the ring-canal and the nerve-ring. 



Reproduction and Development. The sexes of Asterias vul- 

 garis are separate, but externally there is no difference 

 between them. At the time the dissections were made for 

 the drawings, the sexual glands in the specimens were small 

 (Fig. 119, 12; Fig. 121, 18). As the first of June approaches, 

 the ten sexual glands increase in size until they occupy all the 

 available space in the body-cavity of the arms. The ovaries 

 of the females, when they contain ripe eggs, are bright orange 

 in color ; the spermaries of the males are light cream color. 



