The Animal Kingdom - 639 



tries; and some of these are found in cattle 

 and domestic animals, as well as in man. 

 Generally, the adult flukes are found cling- 

 ing to the walls of the large veins of the 

 abdomen; but the eggs leave the human 

 body via the urine or feces. Various snails 

 serve as intermediate hosts for the sporocyst 

 stages and the fork-tailed actively swimming 

 (cercaria) larvae gain entrance into another 

 human host by penetrating the skin of a 

 bather, or via the drinking water. Typically 

 the males and females of Schistosoma are 

 never separated from each other. The broadly 

 flattened male is longitudinally folded, form- 

 ing a groove that encloses the very slender 

 threadlike female. 



Class 3. The Cestoidea (Tapeworms). (Fig. 

 32-14). Some of these elongate parasitic flat- 

 worms may measure as much as 75 feet in 

 length. As the common name implies, the 

 Cestoidea display a flattened tapelike form. 

 As adults, all live in the intestinal tracts of 

 various vertebrates (Fig. 32-17). Here they 

 attach themselves to the gut wall by means 

 of (usually) four suckers and a ring of for- 

 midable chitinous hooks (Fig. 32-14). The 

 headlike attaching section of the worm, 

 which bears the hooks and suckers, is called 

 the scolex; and the rest of the body is made 

 up of sections (not equivalent to the true 

 segments, or somites, of higher animals), 

 which are called proglottids. The proglottids 

 near the scolex are relatively small and im- 

 mature. New ones are continually being 

 budded off from the scolex. Thus the pro- 

 glottids farther away from the scolex are 

 older, larger, and swollen with eggs. Food 

 substances are absorbed directly into the 

 worm's body from the surrounding intesti- 

 nal contents of the host; that is, the tapeworm 

 itself does not have any digestive tract. In 

 fact, most of the organ systems tend to be de- 

 generate, except for the reproductive system, 

 which is highly developed. In each ripe pro- 

 glottid there are dozens of small testes, at 

 least one large ovary, and a complex system 

 of accessory reproductive structures. Flame 

 cells are present, connected to two longi- 



tudinal excretory ducts that extend through 

 the successive sections. Also there are six 

 strands of nervous tissue that pass length- 

 wise through the proglottids. 



Cestoidea, compared to Trematoda, have a 

 relatively simpler life cycle. This is indicated 

 in Figure 32-17, which gives the cycle of the 

 pork tapeworm, Taenia solium, a rare para- 

 site of man. Ripe proglottids, swollen with 

 eggs (often self-fertilized) pass from the 

 human intestine with the feces (Fig. 32-17). 

 Before it is liberated from a proglottid, each 

 egg develops into a six-hooked embryo, the 

 onchosphere, which lies inside a tough en- 

 velope (Fig. 32-17). The juices of the pig's 

 intestine digest away the envelopes, liberating 

 the active hooked larvae. These penetrate 

 into the blood stream of the pig, and the 

 blood carries them to the muscles. Here each 

 larva encysts and undergoes metamorphosis 

 into a cysticercus, or bladder worm (Fig. 

 32-17). The bladder worm possesses a very 

 small inverted scolex, which lies near the 

 center of a fluid-filled bladder; and the blad- 

 der, in turn, is enclosed by a cyst wall. If 

 incompletely cooked (pink) pork, derived 

 from an infected hog, is eaten by a man, the 

 cyst wall digests away, the bladder everts, 

 and the scolex of the young Taenia attaches 

 itself to the intestinal wall. The young worm 

 then develops rapidly. In four to five weeks 

 it may become an adult, about 20 feet long, 

 which will produce thousands of ripe egg- 

 laden proglottids in the next generation. 



The life cycles of various other tapeworms 

 also have been worked out. Perhaps the most 

 important of these are: (1) Taenia saginata, 

 the beef tapeworm (adult stage in man; larval 

 stages in beef muscle; length up to 75 feet); 

 (2) Dibothriocephalus latum, the fish tape- 

 worm (adult stage in man and other fish- 

 eating mammals; one larval stage in Cyclops 

 and other small fresh-water Crustacea; inter- 

 mediate stage in muscle (flesh) of carp, perch, 

 and other fresh-water fish; length up to 30 

 feet); (3) Dipylidium caninum, the dog tape- 

 worm (adults in dogs, cats, and (rarely) man; 

 larval stage in certain lice and fleas; length, 



