FLATWORMS 



radially symmetrical anterior organ of attachment has little resemblance to 

 other flatworms, but examination reveals homologies that clearly place these 

 worms with the trematodes and the turbcllarians. The structure of the 

 cestode is, however, more specialized in relation to parasitism than is the 

 structure of the trematode, when each is compared with the free-living 

 turbcllarians. Not only is the epidermal layer lacking, as in the majority 

 of trematodes, but the cestode has no digestive tract and no endoderm cells, 

 even in its development. The reproductive and excretory systems establish 

 the relationship, however, since these parts are unique and fundamentally like 

 those of other Platyhelminthes. The apparent dissimilarity between the non- 

 segmented fluke or planarian, on the one hand, and the segmented tapeworm, 

 on the other, is not so important as it might seem. Segmentation does not 

 occur in the cestode subclass Cestodaria,- and in the subclass Eucestoda there 

 are some species in which segmentation is lacking or little developed. The 

 segmentation is in fact only a reduplication of the part of the individual 

 containing the reproductive organs and is not entirely comparable with the 

 segmentation of such a truly metameric form as the earthworm. The segmen- 

 tation of the tapeworm may be regarded simply as a device for increasing the 

 reproductive potential. 



The Subclass Cestodaria. These non-segmented tapeworms have an 

 organ of attachment resembling the head-like scolex of the Eucestoda, but 

 they lack the segments, or proglottids, of the true tapeworms, and they have 

 only one set of reproductive organs instead of many sets (Fig. 11.16). Like 

 the Eucestoda, they are covered by a cuticle instead of an epidermis, and they 

 have no mouth or digestive tract. A characteristic ten-hooked larval stage is 

 present in the life cycle. These worms have been compared with trematodes 

 minus a digestive tract, and it has been suggested that they may represent an 

 intermediate stage in the evolution of tapeworms from flukes. From several 

 lines of evidence it now appears that the trematodes and cestodes probably 

 had an independent origin from some free-living group, rather than that the 

 trematodes gave rise to the cestodes. The cestodarians may, in fact, repre- 

 sent an early stage in the evolution of the Eucestoda; alternatively, they might 

 be considered as degenerate eucestodes. 



The Subclass Eucestoda, This subclass includes the great majority of 

 tapeworms. The individual consists of a scolex, which bears the organs of 

 attachment such as suckers and hooks; and of a segmented bodv, or strobila, 

 composed of varying numbers of proglottids (Fig. 11.17). The adult animal 

 lives in the intestine of its host, attached by the scolex and with the 

 strobila extending into the intestinal cavity. The firm hold of the scolex 

 enables the parasite to maintain its position despite the movement of food 

 along the host's intestine. Adult tapeworms vary in size from such tiny 

 forms as Echinococciis granulosus, which when fullv mature is only a few 

 millimeters long and consists of a scolex with four proglottids, to others 

 .such as Taenia saginata, which may be 4 to 10 yards in length and have 

 thousands of proglottids. With very few exceptions, all adult tapeworms are 



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