376 Animal Biology 



chyma (par -eng' ki ma) (Gr. para, beside; engchyma, infusion) is a 

 loosely organized tissue between the alimentary tract and the body wall 

 in which the internal organs are embedded, there being no true coelom. 



Motion and Locomotion. — The body consists of three layers of mus- 

 cles: circular, longitudinal, and diagonal (oblique). 



Ingestion and Digestion. — A mouth lies in the center of a muscular, 

 disk-shaped anterior (oral) sucker. A ventral sucker serves for attach- 

 ment. The mouth leads into a short pharynx, the latter connects with 

 the esophagus, and the esophagus leads to the intestine, with its two 

 main branches and numerous smaller ones. The digestive cavity serves 

 as a gastrovascular cavity for digestion and circulation. 



Circulation.— There is no special circulatory system, although circu- 

 lation is accomplished by the digestive tract (gastrovascular cavity). 



Respiration. — Respiration occurs through the general body surface. 

 Being parasitic, the oxygen needs are probably not great. 



Excretion. — The external excretory pore (nephridial opening) is lo- 

 cated at the posterior end of the body. There is only one main excretory 

 canal with its numerous branches which extend to all parts of the body. 

 Undigested materials may be eliminated through the mouth. 



Coordination and Sensory Equipment. — The nervous system consists 

 of a brain (nerve ring) in the anterior part of the body. Two longitudi- 

 nal nerves extend posteriorly from the brain. The general structure 

 resembles that in Planaria. 



There is no special sensory equipment in the adult stage which is 

 parasitic within the body of the host. 



Reproduction. — The external genital opening is located between the 

 mouth and the ventral sucker. Both male and female reproductive or- 

 gans are present in the adult fluke; hence, it is mo?iecious (hermaphro- 

 ditic). Their arrangements may be studied in Fig. 180, D. 



The various stages in the life cycle in the two hosts may be studied in 

 Figs. 181 and 374. One adult fluke may produce over 500,000 eggs, and 

 200 flukes in the bile ducts of a sheep liver may thus produce 100,000,000 

 eggs. The eggs pass from the sheep liver to the intestine where they are 

 passed with the feces. When in water, an egg produces a ciliated larva 

 called a miracidium (mir a -sid' i um) (Gr. meirakion, stripling). The 

 latter swims until it bores into the body of a certain species of fresh- 

 water snail (Lymneae). In two weeks it changes to a saclike sporocyst 

 (spor'osist) (Gr. sporos, spore; kystis, sac). Each of the numerous 

 germ cells within a sporocyst develops into a second type of larva called 



