Flatworms and Roundworms 381 



smooth, unsegmented body with somewhat pointed ends. The sexes are 

 separate, the female being larger (five to eleven inches long) ; the male is 

 usually smaller with a bend in the posterior part of the body. Because 

 the two sexes can be distinguished by means other than the sex organs, 

 they are said to illustrate sexual dimorphism (di -mor' fizm) (Gr. di, 

 two; morphe, form). A pair of broad lateral lines on either side point 

 out the pair of excretory tubes beneath. No cilia are present on the out- 

 side of the adults (Fig. 184). 



Integument. — The external, transparent cuticle is usually smooth and 

 glistening, with very fine striations. A cellular hypodermis lies beneath 

 the cuticle. Between the body wall and the digestive tract is a cavity 

 which contains large, giant cells loosely arranged to form mesenchyme 

 tissue (mes' eng kime) (Gr. mesas, middle; engcheim, pour in). The 

 latter contains spaces known as vacuoles, but there is probably no true 

 coelom. 



Motion and Locomotion. — Muscle cells in the body walls may cause a 

 limited amount of locomotion as the worms are moved along by the 

 intestinal contents of the host. 



Ingestion and Digestion. — The mouth at the anterior end has one 

 dorsal and two ventral lips which are finely toothed and which bear 

 nipplelike papillae (pa-pil'i) (L. papilla, nipple). A straight pharynx 

 connects with the muscular, sucking esophagus which joins a straight, 

 nonmuscular intestine, ending in the arms at the posterior end of the 

 animal. The muscular esophagus sucks fluids from the intestine of the 

 host. The posterior part of the intestine is the rectum (rek' tum) (L. 

 rectus, straight) . 



Circulation. — Because of the slender body and the absorption of foods 

 through the long intestine, 7io special circulatory system is required. 



Respiration. — The parasitic habits^ and the comparative inactivity 

 preclude any need for a special respiratory system. 



Excretion. — The pair of longitudinal excretory tubes embedded in the 

 pair of lateral fines open to the exterior by one excretory pore on the 

 ventral surface near the anterior end. There are no flame cells. Un- 

 digested materials may be eliminated by the intestine. 



Coordination and Sensory Equipment. — A ring of nervous tissue en- 

 circles the esophagus and gives off a large dorsal nerve cord and a large 

 ventral nerve cord. The two cords may be connected by other nerve 

 rings. There is no special sensory equipment. 



