152 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 cesophagus, 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. 
