THE FLA TWO RMS TAPE WORMS 



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THE FLATWORMS Class Platyhelminthes 



The flatworms are characterized by the absence of a distinct vascular system, 

 and by the alimentary canal being either absent or with no posterior outlet. A 

 nervous system is developed, consisting either of a network of nerves, or sometimes 

 of a distinct brain and lateral cords. The excretory organs are composed of fine 

 tubules opening to the exterior, and the body cavity is reduced to a set of slits in 

 the tissues. For the most part the sexes are united in one individual. Sometimes 

 a sexual reproduction occurs, accompanied by an alteration of generations. There 

 are three orders of flatworms, namely, the tapeworms, the trematodes, and the tur- 

 bellarians. 



TAPEWORMS Order CESTODA 



The members of this extensive group are internal parasitical worms with the 

 body divided into a number of segments. There is no trace of an alimentary canal, 

 nutriment being obtained by the absorption of juices through the entire surface of 

 the body. The head is furnished with suckers, or hooks, or both, by means of 

 which the worm adheres to the walls of the intestine of the host it infests. The 

 nervous system consists of a ganglion in the head, and a cord on each side. As a 

 well-known example, we may take Tcenia saginata, one of the human tapeworms; 

 and since its structure and the phases through which it passes in the course of its 

 development are thoroughly known, a detailed discussion of its characteristics will 

 serve as an introduction to the study of the group. In its mature state this worm, 

 which spends this stage of its existence in the 

 human intestines, may reach a length of five or 

 six yards. Its head is relatively minute, being 

 only about equal to that of an average pin in 

 size, and furnished with four suckers, by means 

 of which the creature adheres firmly to the 

 walls of the intestine. The head is followed by 

 a narrow piece called the neck, which gradually 

 passes posteriorly into the trunk. It is not 

 jointed, but where it merges with the trunk it 

 becomes marked by shallow grooves, growing 

 deeper and deeper as they recede from the head, 

 until ultimately they divide up the body into a 

 chain of flattened, square or oblong segments, of 

 which there may be many hundreds. Each seg- 

 ment is called a proglottis, the whole series be- 

 ing termed proglottides. The muscular system 



is fairly well developed, and consists of fibres running lengthwise throughout the 

 segments and across from side to side, and of others passing from the upper to 

 the lower walls. By means of these muscles the worm is able to shift at will its 



PIUDIUM I,ARVA, WITH NEMERTlNE 

 WORM DEVELOPING INSIDE. 



