if 



372 TEXTBOOK OF ZOOLOGY 



Internal Structure and Metabolic Activities. — In small cleared 

 specimens the internal organs are easily observed. The notochord 

 extends the length of the body as a slender rod of vacuolated cells 

 which are filled with fluid to give it turgor or stiffness. Immediately 

 dorsal to this rod is the nerve cord, which also runs the length of the 

 body. It has a small central canal or neurocoele extending length- 

 wise through it and is dilated at the anterior end to form the cerebral 

 vesicle or rudimentary brain. A mass of dark pigment is located at 

 the anterior end which is known as the eyespot. There are smaller 

 pigment bodies distributed along the length of the cord. These 

 are thought to be sensitive to light. The nerve cord gives off nerves 

 to the organs of the body. The two anterior ones are paired, but 

 those behind the cerebral vesicle alternate on the two sides. There 

 are dorsal sensory nerves going to the skin and ventral motor nerves 

 going to the myotomes. There are sensory cells in the skin, oral 

 tentacles, and velar tentacles. 



The circulatory system does not include a heart, but the blood 

 is moved by the contractions of a ventral aorta, which branches to 

 form the afferent branchial arteries to the gills. Here these vessels 

 branch into capillaries, providing aeration for the blood. These capil- 

 laries converge to form the efferent branchial arteries which lead dor- 

 sally to join the paired dorsal aortae. The dorsal aorta extends pos- 

 teriorly to the tip of the body giving off numerous branches to myo- 

 tomes and internal organs along the way. The posterior direction of 

 the flow of the blood is just opposite to that in the dorsal vessel of the 

 earthworm. The subintestinal vein receives the blood from the in- 

 testine and continues anteriorly to the liver as the hepatic portal vein. 

 The hepatic vein collects from the liver and leads forward as the 

 ventral aorta. The blood in the subintestinal and hepatic portal 

 veins is laden with dissolved nutriment. The blood in these ventral 

 veins flows from posterior toward the anterior (Fig. 216). 



Digestive System. — A current of water is carried into the mouth 

 by the ciliated bands on the inner surface of the oral hood. These 

 cilia form what is called a wheel organ because of their rotary motion. 

 Surrounding the mouth is a membranous velum to which are attached 

 twelve velar tentacles, which fold across the mouth and serve as a 

 strainer to hold back the coarser particles, as well as being sensory. 

 The mouth leads to the large, barrel-shaped pharynx. The gill slits 

 are clefts in the lateral walls of the pharynx. The number of clefts 



