154 THE BIOLOGY OF THE ANIMAL 



part of the abdomen will now be seen to be filled 

 with a mass of muscle, through the middle of 

 which runs a deep groove. In the groove are the 

 superior abdominal artery with its side branches 

 and the intestine. With scissors divide the 

 branches of the blood-vessel as far out at the 

 side as possible, cut the intestine across just in 

 front of its dilated end, and lay both blood-vessel 

 and intestine over at one side. Turn forward 

 the glands and other organs covering the ante- 

 rior end of the muscle. Cut vertically through 

 the mass of muscle at the line of junction of the 

 fifth and sixth segments, and carefully work it 

 out of the shell, taking precautions not seriously 

 to injure the nerves lying below the muscle. 



How does it compare in size with a and 5? 

 Of how many smaller masses is the muscle com- 

 posed ? In what direction do its fibres run ? To 

 what part of the segments are they attached ? 

 Do the fibres extend into the pleural region of 

 each segment ? To what is the anterior end of 

 the muscle attached? For what purpose is the 

 deep median groove? "What motion does the 

 muscle produce ? How do you account for its 

 size? 



d. The adductor muscle of the mandible. — Lay 



the carapace back on the body, note the position 

 of the cervical groove, and look directly in front 

 of it on the inside for the origin of a fan-shaped 

 muscle, which tapers to a tendon and runs ob- 

 liquely downward to the mandible. Note the 

 direction of the muscle. Compare the length of 

 the muscular portion with that of the tendinous 



