156 ESSENTIALS OF ZOOLOGY 



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 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 hrancliial 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 those from the opposite side in forming the dorsal aorta. 

 The dorsal aorta extends posteriorly to the tip of the body giving 

 off numerous branches to myotom.es 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 suhintestinal 

 vein receives the blood from the intestine 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 

 suhintestinal and hepatic portal veins is laden with dissolved nutri- 

 ment. The blood in these ventral veins flows from posterior toward 

 the anterior. 



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

 by the ciliated bands on the inner surface of the oral hood. Sur- 

 rounding 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. These open into the 

 atrial cavity which surrounds the pharynx and other visceral organs. 

 In the midline of the roof of the pharynx is an inverted trough, the 

 hyperhranchial groove, which is ciliated. In the floor of the pharynx 

 is the hypohranchial groove in whose Avails is located the glandular 

 endostyle, capable of secreting mucus. The endostyle functions on 

 the same plan here as in tunicates. The strings of mucus entangle 

 the food particles and are moved anteriorly, and then by two peri- 

 hranchial grooves are carried dorsally to the hyperhranchial groove. 

 The cilia here move the mass back to the intestine. A blind, finger- 

 like diverticulum of the intestine, the liver or hepatic caecum, ex- 

 tends anteriorly from its connection on the anterior part of the 

 intestine to lie on one side of the pharynx. This organ is a digestive 

 gland and empties a digestive juice containing enzymes into the 



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