416 The Animal Kingdom 



THE CLASS CESTODA 



Another class of phatyhelminthes that is entirely parasitic is that 

 containing the tapeworm and known as the Cestoda. The adults are 

 always parasitic in the intestines of vertebrates. None possesses a di- 

 gestive tract as they absorb carbohydrates directly from the partially 

 digested food of the host. It is believed that the other food require- 

 ments may be absorbed from the mucous membranes of the host. The 

 life cycle of the tapeworms involves two hosts, with the larval stage 

 being in an arthropod or in another vertebrate. 



The Anatomy of an Adult Tapeworm. — An individual tape- 

 worm consists of an anterior scolex and a chain of segments or pro- 

 glottids. The scolex is highly modified for attachment to the host and 

 may have either suckers, hooks, or both. Behind the scolex is a 

 narrow neck region where the proglottids are constantly being formed. 

 Most of the worm consists of a linear series of these segments or pro- 

 glottids. The young proglottids which are present just posterior to 

 the neck are largely undifferentiated. Behind these, the proglottids 

 gain in sexual maturity until those of the middle area are sexually 

 mature and have a complete set of organs. Beyond this, the organs 

 degenerate until the posterior gravid proglottids contain little more 

 than the greatly enlarged uterus with its masses of eggs. These distal 

 segments gradually break off and pass out with the feces of the host. 

 Some of these chains of proglottids may be many feet in length. 



There is some nervous coordination in the whole tapeworm. This 

 is accomplished by means of a few ganglia and commissures in the 

 scolex. From these ganglia, longitudinal nerve cords pass the entire 

 length of the worm. When an individual segment does pass out with 

 the feces, it shows ability to move and some sensitivity to stimuli. 



In most details, the excretory system is quite similar to that of 

 the flukes. It is composed of two pairs of lateral tubes joined at the 

 posterior end of the proglottid by a transverse tube. From these main 

 canals smaller tubules pass into the parenchymatous tissue and branch 

 into many smaller tubes, each ending in a flame cell. 



Other than these few structures, the organs of each proglottid are 

 entirely concerned with reproduction. In each proglottid there is a 

 complete set of male and female organs. 



The male system consists of many small scattered testes, each of 

 which is connected by very tiny tubes to the main sperm duct, the vas 



