6o 



Make a drawing showing the complete musculation of two 

 successive segments. 



The circulatory system. — Cut a second specimen open 

 longitudinally along the median line of the venter, reserving 

 the first specimen for some later work. Pin out the cut 

 edges. Note again the general disposition of the body or- 

 gans so far examined. Examine again the reproductive or- 

 gans ; the specimen may be of the other sex from that pre- 

 viously studied. Remove the alimentary canal. 



The dorsal vessel or heart is a slender, delicate, membranous 

 tube composed of a number of successive parts or chambers 

 lying along the median line of the back. To see this well 

 cut out the median dorsal strip of body-wall carrying the 

 heart and transfer it to a glass slide. Cover with water and 

 examine first under the simple microscope and then under 

 the low power of the compound microscope. It will be dis- 

 tinctly seen that toward the middle of each segment from 

 the last to the third inclusive, the tube becomes dilated, 

 forming a chamber, and in most of these chambers, except 

 the last, a pair of internal valves may be seen. From the 

 most anterior chamber a straight, tapering tube, the aorta^ 

 runs forward into the head, where it ends by dividing into 

 branches which can be followed for but a short distance, ap- 

 parently fading out completely. There are no other blood- 

 vessels in the body. 



On each side of each chamber may be noted a fan-shaped 

 group of very delicate muscle fibers, apparently attached to 

 the sides of the chamber, and called the wing muscles of the 

 heart. The convergent outer ends of these muscles are at- 

 tached to the body-wal! on the line of the median constric- 

 tion in each segment. These muscles really lie in a thin 

 membrane which forms a sort oi pericardial membrane enclos- 

 ing a sinus on each side of the heart and stretching across 

 from these lateral sinuses to the body-wall as a septum or 



