370 



THE ANIMAL KINGDOM 



A pair of gonads (Fig. 19.6 C) are located one on each side of the 

 gastric gland in the base of each ray. They hang free in the body cavity 

 except where each is attached by a short duct to a reproductive pore 

 opening externally between the bases of adjacent rays. In the spring the 

 gray testes or orange ovaries are prominent, and large numbers of 

 gametes are released in June. Fertilization is external. 



1 67. Class Asteroidea, the Starfish 



Asterias forbesi is a member of this class. Most starfish have five rays 

 and a relatively small disc. In some, however, the disc is large relative to 

 the rays and the body is pentagonal rather than star-shaped (Fig. 19.9). 

 In one genus, Leptasterias (Fig. 19.9), the animals have six rays. In other 

 starfishes the number of rays may be as high as 25 or 50. Usually the 



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Figure 19.9. Other members of the Asteroidea. A pentagonal starfish, Culcita (left). 

 A starfish with six rays, Leptasterias (center). A starfish with 12 or more rays, Crossaster 

 (right). (After Hyman.) 



number of rays in a species is variable if it is greater than seven. In all 

 cases studied where the number exceeds five the embryo first develops 

 five rays and adds the others later. 



Many starfish live and feed like Asterias. Some eat only small bi- 

 valves and other organisms which are swallowed whole into the cardiac 

 stomach. Many of the large species one to three feet in diameter feed 

 primarily upon other echinoderms. 



1 68. Class Crinoidea, the Sea Lilies 



Sea lilies are echinoderms attached to the bottom by a stalk (Fig. 

 19.5 E). The mouth is directed upward, with the anus located to one side 

 on a small projection. The five rays are usually branched to form a 

 graceful pattern. Ciliated grooves in the epidermis extend out from the 

 mouth along the upper surfaces of all the rays and branches. Each groove 

 is flanked on both sides by tube feet, but these lack suckers and are 

 covered with numerous tiny sensory papillae. The movement of the tube 

 feet pushes tiny organisms and food particles against the ciliated groove, 

 which is covered with a mucous secretion that is continually swept 

 toward the mouth. In this way food is trapped and swallowed. Move- 



