INTERNAL STRUCTURE 81 



lengthens the body and the contraction of the longitudinal 

 muscles shortens it. The seta3 are connected with the 

 longitudinal muscles. By pointing the set;e backward 

 and bracing them against the ground, the worm can 

 push itself forward. By pointing the setae forward tin- 

 worm can instantly change the direction of its movement. 



LABORATORY STUDY 



One of the annelids should be studied with some care, as an illustration 

 of an invertebrate animal. How do you determine the anterior and 

 posterior ends ? Dorsal and ventral surfaces ? The number of segments '.' 

 Compare several worms. The back region of the worm shows the most 

 variation because new segments are being added. Where are the setae ? 

 How does the earthworm move ? Place it on a glass. The front region 

 of the body is most sensitive to touch. Test it. 



69. Internal Structure of Earthworms. — This is shown 

 diagrammatically in Figure 80. The internal structure 

 consists of an outer tube, 

 the body wall, and an 

 inner tube, the digestive 

 tube. The space be- 

 tween the body wall and 



f"iji yr 



tdui wen 



digestive tube is known FlGURE 8 o.- Diagram. 



as the body cavity or T h e organs of earthworm from the side. 



coelome (se'liim : Greek, 



koilos, hollow). Thin sheets of membrane pass from each 



furrow between the segments to the digestive tube. 



Beginning at the front end the digestive tube is given 

 certain names for each distinct region, as follows : the 

 mouth cavity; the pharynx (far'inks), with its thick 

 muscular walls ; the esophagus (e-sof'a-giis), thin- walled 

 and small ; the crop, a wide pouch ; the gizzard, where 

 food is ground; and the stomach-intestine, a large, thin- 

 walled tract extending through the last tw r o thirds of the 

 length of the worm. 



