142 INVERTEBRATE ZOOLOGY 



the apparatus can be thrust down and partly out of the mouth; 

 also the retractor muscles which pass from the lower part of it to 

 the tall inner projections of the shell. 



Exercise 5. Draw a diagram showing the dentary apparatus in 

 the body and its muscular attachment to the shell. 



The oesophagus, after leaving the dentary apparatus, passes to 

 the elongated stomach ; this lies close to the body-wall, to which 

 it is attached by means of a mesentery. Carefully follow the 

 stomach, breaking away the wall if necessary, as it winds 

 around the inner surface of the shell. From the stomach a 

 short intestine passes to the anus. In making this dissection, 

 keep the animal immersed in clear water; 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 and 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 

 the 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 apparatus is made up of a complicated system of 

 smaller plates. 



Exercise 7. Make a drawing of the dentary apparatus. 



