104 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



c) Structure of the intestinal wall: Examine with the high power. 



(1) Chlorogogue cells: The outer lay^er of the intestine consists of very large, 

 irregular, somewhat pear-shaped cells which are modified peritoneal cells. They 

 are named the chlorogogue cells. . 



(2) Longitudinal muscle layer: At the inner ends of the chlorogogue cells a 

 row of circles, cross-sections of a very thin layer of longitudinally arranged muscle 

 cells, appears. 



(3) Circular muscle layer: A thin layer of circularly arranged muscle cells 

 lies just within the preceding. 



(4) Vascular layer : The blood vessels of the intestine, branches of the dorsal 

 and ventral blood vessels, run in a definite layer in the wall of the intestine just 

 within the circular muscles. Here they form a rich network, appearing in the 

 section as irregular spaces, uniformly stained. 



(3) Lining epithelium: The innermost layer of the intestinal wall, lining the 

 cavity, is an epithelium, consisting of long, slender, ciliated, epithelial cells. 

 This layer is almost as wide as the layer of chlorogogue cells. The nuclei are 

 near the bases of the cells. 



Draw a portion of the intestinal wall in great detail to show these layers. 



d) Structure of the nerve cord: Examine with the high power. The nerve 

 cord is somewhat oval in outline and is covered externally by a sheath consisting 

 of peritoneum, connective tissue, muscles, and blood vessels. Of these the more 

 conspicuous ones are the subneural vessel on the median ventral side and the 

 paired lateral neural vessels, one on each side of the subneural vessel. 



The cord is more or less distinctly divided into two halves by a median parti- 

 tion. On the dorsal side of the cord are three large clear areas, the giant fibers, 

 believed to be nerve fibers which run for long distances in the worm. Each is 

 surrounded by a thick sheath. In the lateral and ventral portions of the cord 

 may usually be seen several large pear-shaped nerve cells, each with a large 

 nucleus and nucleolus. The nerve cells are more abundant if the section has 

 happened to pass through one of the ganglionic swellings of the cord. The rest 

 of the cord consists of cross-sections of nerve fibers, appearing like an open 

 network. 



Draw the section of the nerve cord. 



e) Longitudinal section of the earthworm: Study and identify the parts already 

 seen in the cross-section. Note particularly the septa. 



4. General considerations on the earthworm. What system is present in 

 the earthworm which was lacking in Planaria? What systems which the frog 

 possesses are lacking in the earthworm? How do you think the earthworm 

 compares with the frog with respect to differentiation of organs and systems? 

 What particular system shows the least advance over the same one in Planaria 

 and least resembles that of the frog? In what ways does the digestive tract of 

 the earthworm differ from that of Planaria and resemble that of the frog? In 



