GENERAL ZOOLOGY 



Wall of 

 cyst 



Fluid -filled 

 ^ cavity of cyst 



Sucker 



Fig. 11.20. Portion of a hydatid cyst of Echinococcus granulosus. The outer part of the cyst 

 wail is produced by connective tissues of the host; the inner part is a thin syncytium from 

 which many brood capsules, like the one shown here, arise by budding. Thousands of scoleces, 

 each capable of producing a mature tapeworm, are formed asexually within such capsules in a 

 hydatid cyst. (Redrawn from C. G. Goodchild in F. A. Brown, Jr., et al.. Selected Invertebrate 

 Types, copyright 1950 by John Wiley and Sons, Inc., printed by permission.) 



case of T. solium, this would be the pig. Within the intestine of the proper 

 intermediate host, the eggshell is digested, but the hexacanth survives and 

 w^orks its way, using its hooks, through the mucous membrane and into the 

 blood vessels or lymph channels. It may then be carried to the muscles of 

 the host, where it develops into a form called the bladder worm or cysticercus; 

 this stage consists of a bladder with the scolex and neck of the future adult 

 invaginated from one point on the surface (Fig. 11.19). This bladder worm 

 cannot develop further unless it is eaten by the final host, which in this case 

 is man. Once in the human digestive tract, the scolex is freed by the di- 

 gestion of the cyst and bladder; it then everts, attaches to the intestinal 

 wall, begins a rapid formation of proglottids, and so produces the adult worm. 

 Such an adult may live for some years, forming and detaching thousands of 

 proglottids, each containing countless eggs. 



This life cycle may be summarized as follows: one proglottid produces 

 thousands of zygotes; each zygote develops into one hexacanth; each hexa- 

 canth becomes one cysticercus, which transforms into one adult. Thus the 

 reproductive potential of these tapeworms is increased by multiplying the 

 number of zygotes produced, and not, as in the Digenea, by modifications 



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