A MANUAL OF ZOOLOGY 21 



above are rudimientary. Give reasons for calling them 

 such. .Make a drawing of a gill. Note in general the 

 exoskeleton. Boil a piice' ofTBeTeiosk^leton. Place HCl 

 upon another piece. What is its composition? Remove 

 one of the eye-stalks. Study the surface of the eye. Note 

 on the cornea, the flat areas, or facets. What is the shape 

 of the facets? Make a longitudinal section of the eye. 

 stalk and all. Study with low power. 'Note the spindles, 

 the cr/stalline cones, the optic ganglion,' and the optic 

 nerve. 



By the use of a bristle' or pointed instrument find the 

 opening of the auditory organ among the setae on the 

 bas^l joint of the antennule. Now cut away the under 

 side of the joint and find a transparent sac, the auditory 

 sac. Study carefully with and without the microscope. 

 Study the contents of the sac for sand-like grains, the 

 otoliths. Note also the auditory hairs. The olfactory or- 

 gans consist of setae on the endopodite of the antennule. 



With strong scissors cut through the exoskeleton on 

 the sides from the telson to the rostrum. Carefully lift 

 It off beginning at the abdominal segments. Directly un- 

 der the shell find the epidermis. Notice how it is attached. 

 On either side of the mid-dorsal line of the abdomen 

 directly beneath the epidermis find the extensor abdo- 

 minis muscle. Note its origin and its insertion. To 

 what are these muscles attached?, Remove them. Find 

 the levator abdominis muscle, whose origin is on the cara- 

 pace just above that of the extensor abdominis. Find 

 its insertion. The contraction o( these muscles will pro- 

 duce what motion? The great muscle now seen to fill 

 the greater part of the abdomen Is the flexor abdominis. 

 A deep groove runs through the middle of it containing 

 the superior abdominal artery and the Intestine. As far 

 out to the side as possible cut these arteries, also cut the 

 intestine in front of its dilated end, and turn both back 

 ' to one side. Now work the muscle out of the shell, being 

 careful not to injure the nerves underneath. Find origin 

 and insertion of the muscle. What motion does this muscle 

 ■ produce? Why is it so large? The adductor muscle of 



the mandible has its origin on the carapace in front of 

 *Ua ^arvinal ernnve. Tt is a fan-shaned rniisnle ending in 



