THE THREE-LAYERED ANIMALS 



171 



per sulfate in order to destroy the snails 

 have been developed and this control has 

 had reasonable success. 



CLASS CESTOIDEA 



These are the tapeworms, a group of 

 parasites that the layman has long errone- 

 ously associated with lean hungry adoles- 

 cents. The worm gets its name from its 

 long ribbon-like appearance, a feature that 

 is common to this large and varied group, 

 members of which infect almost all, if not 

 all, vertebrate animals. 



The tapeworm is, perhaps, the most de- 

 generate of all animals, a condition indicat- 

 ing that the association with its host is one 

 of long standino;. At the same time- it is 

 beautifully adapted to its specialized envi- 

 ronment, the vertebrate grut. It is indeed 

 the supreme parasite among parasites. It is 

 provided with excellent hold-fast organs to 

 keep it in place in the gut of the host ( Fig. 

 9-12). All nourishment is received from the 

 ■contents of the gut or from the gut wall di- 

 rectly, so the animal has not bothered to 

 retain even a semblance of a digestive tract. 

 Its nervous and excretory systems are very 

 rudimentary, and its ability to move has 

 been reduced to very feeble contractions. 

 However, it has evolved an extremely elab- 

 orate and prolific reproductive system, an 

 essential feature in its survival since the 

 possibility for any individual egg to reach 

 maturity is very small. Although it has de- 

 generated in other respects, it has gone all 

 out in this one phase of its life, and meas- 

 ured in terms of biological success, the shift 

 in emphasis has apparently been satisfac- 

 tory. 



The common beef tapeworm (Taenia 

 saginata) of man is a typical example of 

 this group (Fig. 9-13). It consists of two 

 principal parts, the head or scolex, and the 

 proglottids, sectional pieces attaching to 

 one another, and growing larger as tliey 

 proceed posteriorlv. The scolex possesses 

 hold-fast organs which make it possible for 



Fig. 9-12. The scolex of the dog tapeworm (Taenia 

 pisiformis). Note the sharp hooks and sucking discs 

 used as attachment organs. 



the worm to maintain its position in the gut. 

 The proglottids, which are budded off from 

 the region just back of the head called the 

 neck, matrn-e as they move progressively 

 posteriorly. The younger proglottids are 

 tlierefore anterior to the older. The mature 

 proglottids, gorged with eggs containing 

 young embryo worms, break away from the 

 worm and pass out of the body in the feces. 

 Because of the close association of cattle 

 and their keepers in certain parts of the 

 country, it is not unusual for the eggs to 

 be picked up by grazing cattle. Once in the 

 gut of this host the egg membranes and 

 shell are digested away and the young six- 

 hooked embryo (hexacanth) emerges. It 

 soon bores its way through the gut wall into 

 a blood vessel where it floats to the muscles, 

 particularly heart and jaw muscles. Here 

 it develops into a bladder and a tiny in- 

 verted tapeworm scolex grows from the 

 wall of the bladder. Beef so infected is 

 said to contain "bladder worms" and is usu- 

 ally unmarketable. If such beef is poorly 

 cooked and then eaten by humans the blad- 

 der worms "hatch." The tiny scoleces evert 

 and become attached to the soft intestinal 

 wall where thev immediately begin bud- 

 ding off proglottids. 



