PRINCIPLES OF ANIMAL BIOLOGY 29 



the intestine. These tubes are the nephridia (singular, nephridium), or 

 excretory organs. Does Hydra possess any definite excretory organs? 



60. Nervous System. In the third somite is a small whitish bi-lobed 

 structure, the brain, resting on the buccal pouch. In the posterior 

 part of the worm push the intestine aside, and note the white nerve cord. 

 How far does it extend forward and backward? The thickenings of the 

 nerve cord are the ganglia. Note the small nerves running out from the 

 ganglia. Find the connection between the brain and the nerve cord in 

 the anterior portion. The connecting cords are called the circum- 

 pharyngeal connectives. 



6h. Muscular System. The longitudinal muscles are visible in the 

 dissection as glistening strands running lengthwise on the inner surface 

 of the body wall. 



Make a drawing of the first 25 somites of the dissection three times 

 natural size, putting in all the organs that can be seen in a dorsal view. 

 Turn aside the seminal vesicles of one side so that the underlying organs 

 can be exposed and included in the drawing. Label all the parts identi- 

 fied and shown and indicate somites 1,5,10, 15. 



6z. Examine prepared slides of cross-^sections of the earthworm under 

 the dissecting microscope. Observe again the ccelom or body cavity 

 directly between the body wall and the intestine. The intestine is seen in 

 the middle of the section. Determine the dorsal and the ventral sides of 

 the section. This may be done by using some of the following organs as 

 landmarks. 



In the intestine note the typhloscle which is an infolding of the dorsal 

 wall of the intestine. On the dorsal side of the intestine is the dorsal 

 blood vessel. Beneath the intestine is the ventral blood vessel, supported 

 by a thin membrane or mesentery seen in the cross-section as a wavy line. 

 Near the ventral blood vessel is the nerve cord. In the ccelom may also 

 be found portions of nephridia and sometimes portions of septa. 



The body wall consists of four distinct layers. Lining the ccelom 

 is a very thin layer of cells, the peritoneum. Outside of this membrane 

 is a layer of more or less feathery appearing structures, the longitudinal 

 muscles. Outside of these is a layer of circular muscles. External to 

 these is the hypodermis. How many layers of cells in it? 



The intestine also consists of four layers. On the inside is a single 

 layer of slender epithelial cells. Outside of this is a circular muscle layer; 

 then a longitudinal muscle layer reduced to a few fibers; and covering the 

 intestine is a layer of thick peritoneal cells. 



Draw an outline figure showing the form and position of the various 

 layers of tissue and other organs in outline, but do not fill in details. The 

 boundaries of the layers and organs are sufficient. Be careful to make 

 this drawing with the dorsal side toward the top of the page. 



Does the earthworm possess germ cells and somatic cells? If so, 



