122 PHYLUM ECHINODERMATA 



remove as little of the shell as possible, and do not remove any 

 of the organs from the body. 



The genital system is similar to that in the starfish. The sexes 

 are separate, but the sexual glands of the male and female are alike 

 in appearance. They consist of five radial, granular masses, which 

 lie in the upper part of the body cavity, each mass communicating 

 with the outside through one of the genital pores. The actual 

 extent of the sexual glands depends upon the sexual condition of 

 the animal. During the breeding season, in the summer, they 

 may almost fill the body cavity. 



Exercise 6. Make a diagrammatic drawing of the digestive and 

 reproductive systems and label all their parts. 



Remove the dentary apparatus from the body and examine it 

 carefully. It is made up principally of five triangular plates called 

 alveoli, the lower ends of which bear the teeth. The alveoli are 

 bound together by short muscles. The base of the dentary ap- 

 paratus is made up of a complicated system of smaller plates. 



Exercise 7. Make a drawing of the dentary apparatus. 



The ambulacral system is similar to that of the starfish. A ring 

 canal surrounds the oesophagus just inside the inner end of the 

 dentary apparatus and is connected with the madreporite by 

 means of the stone canal. This organ is a small tube which lies in 

 contact with the oesophagus and also, in the neighborhood of the 

 aboral body wall, with the rectum. From the ring canal five ra- 

 dial canals pass along the median lines of the rays to their aboral 

 ends, sending off branches to the ambulacral feet. The entire sys- 

 tem of tubes, except the ambulacral feet, is within the body cavity, 

 instead of outside it, as in the starfish. Look on the inner sur- 

 face of a ray for the radial canal. On each side of it observe the 

 row of small vesicles, the ampullae, which are the reservoirs of the 

 ambulacral feet. Determine the exact relation of the ampullae to 

 the feet, and of both to the ambulacral pores in the shell. It will 

 be seen that there are two rows of these latter on each side of the 

 radial canal. Through one of these rows the branch canals pass 

 from the radial canal to the ambulacral feet on the outside of 



