92 Invertebrate Zoology. 



bears several longer setae. External to and above the exop- 

 odite is a small sac, the gill-sac. Above the gill-sac is the 

 broad, paddle-shaped gill. 



Examine the remaining thoracic appendages. Are they 

 all built upon the same plan? What functions do they 

 perform ? Recall the parapodia of Nerds. 



At the anterior end of the abdomen are the external 

 sexual organs. In the female they will be seen to enclose 

 the eggs. In the male a tubular organ is formed. Are the 

 terminal fin-like organs comparable with the thoracic ap- 

 pendages ? 



Make drawings of the specimen as seen from the side, 

 and of the more important and characteristic parts. 



Digestive System. Open a specimen along the mid- 

 ventral line, and observe the course of the alimentary tract. 

 It extends as a tube from the mouth to the anus. Does 

 the anus open dorsally or ventrally? Note any local en- 

 largements of the alimentary tract. 



Make a drawing of the tract. 



The Circulatory System consists of an elongated dor- 

 sally lying tube, the heart. It is provided with numerous 

 lateral slits t a pair for each surrounding segment, and 

 extends from near the anterior end of the thorax to the 

 posterior part of the abdomen. The blood enters the heart 

 through the lateral slits, and is forced out through the 

 anterior aorta into the sinuses of the body, finally reaching 

 the lamellated gills, where it is aerated. After being purified, 

 a system of channels conveys the blood to a pericardial 

 sinus, from which it passes through the slits or ostia into the 

 heart. 



