404 GENERAL ZOOLOGY 



There is no digestive tract, nutrition being absorbed through 

 the body wall. The outer covering is a nonciliated cuticle 

 capable of resisting the action of digestive juices (Fig. 231). 



Example: Taenia solium, the pork tapeworm, is found as an 

 adult in the human intestine, where it reaches a length of from 

 4 to 10 meters. It attaches itself by means of hooks and suck- 

 ers. A small six-hooked larva develops from the fertilized egg 

 and is expelled. The larva must then enter the stomach of the 

 pig, where it bores through the stomach wall, and finds its way 

 to the skeletal muscles. Here the larva develops into an oval 

 cysticercus (9 by 5 mm.) and remains as a cyst until the infected 

 tissue is eaten by man. In the cavity of the cysticercus the 

 scolex is developed in an inverted position, and when the cyst 

 wall is dissolved by the gastric juice, the scolex is protruded like 

 the finger of a glove. In the intestine the scolex attaches itself 

 and develops proglottids from its posterior end (Figs. 231, 232, 

 and 233). Taenia saginata, the beef tapeworm, has a similar 

 life history, except that the intermediate host is the ox. It is 

 the commonest tapeworm in the United States. 



PHYLUM 6— NEMERTEA 



Nemertea (from Nemertes, one of the sea nymphs of Greek 

 mythology) is the name of a group of ciliated, unsegmented 

 worms, mostly marine, often included in the Phylum Platyhel- 

 minthes. The body is wormlike, ovoid, or circular in cross 

 section and varies in length in different species from a few milli- 

 meters to 6 meters or more (30 meters in the case of Lineus 

 longissimus) . The alimentary canal, beginning with a mouth 

 on the ventral surface near the anterior end, is a pouched tube, 

 extending the length of the body and terminating in an anal 

 opening. There is no body cavity, the space between the intestine 

 and the body wall being occupied by a gelatinous parenchyma. 

 A simple blood circulatory system is represented by two or more 

 longitudinal vessels, connected with each other and with blood 

 sinuses in the tissues. At the anterior end is a four-lobed cephalic 

 ganglion above the alimentary canal, from which nerves extend to 

 various parts of the body. The excretory system consists of 

 flame cells, connected by tubules with two longitudinal canals 

 opening on the surface of the body. Numerous simple eyes, 

 ocelli, occur, sometimes along the sides of the body. Some also 



