156 THE BIOLOGY OF THE ANIMAL 



you explain its large size and the connection to it 

 of so many muscle fibres ? Why should this be 

 called the adductor muscle ? On the inner side 

 of the base of the propodite find another tendon 

 to which are attached muscle strands. Compare 

 this tendon and muscle in all respects with the 

 first, and give reasons for the differences found. 

 Why is this muscle called the abductor ? Notice 

 that each tendon is an ingrowth of the exoskele- 

 ton at the base of the dactylopodite. 



Peel what is left of the muscle mass out of the 

 two segments, leaving the epidermis attached to 

 the exoskeleton, and note the honeycomb appear- 

 ance of the epidermis, especially near the base of 

 the propodite. What causes this appearance ? 



g. The microscopic structure of muscle. — From 

 a recently killed specimen take a small piece of 

 muscle, lay it on a slide in a drop of normal salt 

 solution, tease the muscle carefully with fine- 

 pointed needles, and examine under the low, then 

 the high power. What is the structure of a mus- 

 cle strand ? Of one of the constituent fibres ? 

 Are the fibres striated ? Draw. 



Stain in dilute acetic acid carmine and note 

 the results. 



C. — The Circulatory System. 



Lying at the anterior end of the superior abdominal 

 artery look for 



a. The pericardial sinus. — What is the shape of 

 the sinus ? With scissors divide it along the me- 

 dian dorsal line, and carefully fold back the flaps 

 thus made. What is the nature of its wall? Look 



