GENERAL ZOOLOGY 



14. The size of these glands indicates an important function, but the role of 

 their secretion is obscure. They produce spherules and concretions of calcium 

 carbonate, which are emptied into the lumen of the esophae^us. These may 

 serve as a means of neutralizing excess acidity arising from ingested food, 

 or as a mechanism of eliminating excess carbonate from the blood. The crop, 

 as in other animals, is a storage place for food awaiting digestion; the 

 gizzard serves to reduce the size of food particles. The stomach-intestine 

 is the principal region of digestion and absorption. The surface area of this 

 part of the digestive tract is increased significantly by the development of a 

 continuous longitudinal fold, the typhlosole, which hangs down from the 

 dorsal wall of the gut through the entire length of the stomach-intestine. 

 Absorption involves the passage of products of digestion through the wall of the 

 gut and into the blood vessels with which it is richly supplied. 



The coelom of Lumhricus, like that of Nereis, is divided into successive 

 compartments by transverse septa. These compartments are not completely 

 isolated, however; in the median ventral part of each septum there is a large 

 opening, which makes the coelom essentially a continuous cavity and permits 

 the flow of coelomic fluid from one part of the body to another. In Lumbricus 

 longitudinal mesenteries are not well developed, being limited to the mesen- 

 tery which supports the blood vessel ventral to the gut. 



The circulatory system is a surprisingly highly organized, closed set of vessels 

 containing blood colored red by hemoglobin in solution in the plasma. The 

 blood is circulated by peristaltic contractions in the walls of the larger blood 

 vessels. The major features of the arrangement of the vascular system may 

 be summarized as follows: two chief longitudinal vessels are present, the dorsal 

 vessel, lying dorsal to the gut, above the typhlosole, and the ventral vessel, King 

 just beneath the digestive tube. In somites 7 to 11, these two vessels are 

 directly connected by five pairs of so-called ''hearts," which pass around the 

 esophagus. One additional longitudinal vessel, the subneural, lies beneath the 

 ventral nerve cord, bound up within its peritoneal sheath, and a pair of lateral 

 neural vessels also parallel the nerve cord. The ventral vessel distributes blood 

 to the nephridia, the body wall, and the wall of the gut, through specific 

 branches at segmental intervals. From the body wall, blood mav flow 

 through collecting branches into the subneural and lateral neural vessels, to 

 be sent to the dorsal vessel through paired, segmental commissural vessels. The 

 commissural vessels also receive parietal vessels draining the body wall and 

 nephridial vessels from the segmental excretory organs. From the gut wall 

 blood reaches the dorsal vessel through numerous dorsointestinal vessels. 'Fhe 

 dorsal vessel, through which blood flows anteriorlw thus scrxcs as a collecting 

 \essel, although it sends blood to the gut wall through afferent typhlosolar 

 vessels. The ventral vessel propels blood posteriorlv, in general, and is the 

 chief distrifjuting vessel. In the region anterior to the "hearts" these relation- 

 ships are reversed. Capillary networks lie in the tissues between adjacent 

 branches of the distributing and collecting systems. Functionally, the blood 

 flowing to all tissues nourishes their cells, brings oxygen to them, and removes 



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