LOWER ANIMALS 



207 



sites are not really simple creatures. The impression 

 of simplicity is often obtained when too much stress is 

 placed upon structures that often are absent in para- 

 sites. However, as with flukes, parasites may have a 

 high degree of specialization of certain structures, 

 and often a very complex life cycle as well. 



CLASS CESTODA (Tapeworms) 



Diagnosis: strictly parasitic; body divided into 

 proglottids which are not true segments, covered by a 

 cuticle, but no epidermis or cilia; attachment often 

 involves suckers, but also involves adhesive grooves or 

 hooks; no digestive tract in the adult; mostly 

 hermaphroditic and self-fertilizing; internal parasites 

 with indirect development including changes of 

 hosts; adults are vertebrate intestinal parasites 

 (Figure 12.11). 



Tapeworms were among the first animal parasites 

 noted by man. Like the flukes, all members of this 

 class live with other organisms; what is more, all are 

 parasites. Tapeworms have lost all trace of a diges- 

 tive tract, but they have gained certain unique 

 adaptations. For example, feeding is accomplished 



Figure 12.1 1 Life cycle of the pork tapeworm. Taenia: A, adult tape.- 

 worm in human intestine; B, mature egg sac (proglottid) breaks off end 

 of tapeworm, leaves the human body, and is eaten by o pig,- C, bladder- 

 worm stage resides in pig muscle after egg hatches, and hotchling 

 migrates to the muscle; D, blodderworm evoginotes its head when it 

 reaches o human intestine. Survival of this human invading stage is de- 

 pendent upon pork's being incompletely cooked. Under such conditions, 

 the head of the blodderworm grows the segments to form the "tope." 

 The farther the segments ore from the head, the closer the egg sacs ore 

 to maturity. 



solely by absorption through the body wall. The an- 

 terior end, the "head," fastens to the gut wall of the 

 host by means of suckers and quite often an accessory 

 armature of hooks. Behind this "head," or scolex, 

 and its neck is a long body of many pseudoseg- 

 ments, or proglottids. The body is called the strohila. 

 Least developed pseudosegments are nearest the 

 "head," and sexually mature ones are at the posterior 

 end. Pseudosegments bud ofTat the neck as relatively 

 undififerentiated masses of tissue. As these masses 

 are pushed back by the development of others, re- 

 productive structures form within each proglottid, 

 fertilization occurs, and most posterior pseudoseg- 

 ments are practically nothing but egg sacs. The pork 

 tapeworm. Taenia solium, is a good example of the life 

 cycle. Ripe pseudosegments, those containing fertile 

 eggs, break ofT from the posterior end of the worm 

 and pass out with the excrement. The eggs may be 

 freed then or in the intestine of a pig eating con- 

 taminated food. In the pig's intestine, a unique larva 

 emerges and with its six hooks bores through the in- 

 testinal wall of the pig to a blood vessel. At an ap- 

 propriate place the worm bores through the blood 

 vessel wall and emerges into muscle tissue. Here it 

 encysts and, growing within its protective layer, pro- 

 duces a miniature "head" inverted within a capsule. 

 This stage is called a bladder worm. When the pig is 

 slaughtered for meat, this bladder worm remains alive 

 for some time and survives if the cooking is not 

 thorough enough. Emerging then in a human's intes- 

 tine, the "head" is extruded, attaches to the intestinal 

 wall and the parasite begins feeding and growing. As 

 far as man is concerned, this organism is more 

 dangerous as a larva than as an adult. If man eats 

 food contaminated with tapeworm eggs, the burrow- 

 ing of the larva and its subsequent growth may cause 

 great damage and death. The effects of the adult on 

 man are relatively minor. 



NEMERTEA (= NEMERTINEA; 



RHYNCHOCOELA) 



(Ribbonworms) 



Diagnosis: symmetry bilateral; body unseg- 

 mented, slender, soft, and highly contractile, worm- 

 like; no body cavity, but with anus, circulatory sys- 

 tem, and snout, or proboscis, having a tubular cavity 

 above the digestive system; above the mouth the soft 

 proboscis may extend far out or be completely with- 

 drawn like the inverted finger of a glove, leaving only 



