CESTODA 199 



system consists of two cords running down either side of the body 

 with a single similar commissure at either end. But when the animal 

 moves it has the "frilled" organ in front so that is spoken of as the 

 anterior end. 



Order CESTODA MEROZOA 



These are distinguished from the Cestoda Monozoa by the fact that 

 they all have the power of budding and so reproducing asexually, re- 

 sembling in this respect the turbellarian Microstoma lineare. They 

 are all parasitic as adults in the alimentary canals of vertebrates. The 

 adult worm has a scolex which is provided with organs of fixation such 

 as hooks, suckers or folds (Fig. 157). The hooks are borne on a pro- 

 jection at the top, called the rostellum^ while the suckers are on the 

 sides of the scolex. The scolex is usually buried in the intestinal 

 mucosa of the host. Behind the scolex comes the neck^ the most 

 slender portion of the body, which may or may not be sharply 

 marked off from the scolex. It is in the neck that asexual repro- 

 duction occurs, fresh segments being continually cut off and, as 

 they grow larger, pushed by the formation of new segments away 

 from the scolex. The segment so formed is called a proglottis. The 

 proglottis is not truly comparable with the new individuals produced 

 in Microstoma lineare. Through each proglottis runs the excretory 

 canals and the nervous strands which are common to all (Fig. 156). 

 The proglottis when first cut off from the neck region is devoid of 

 generative organs, but these develop as it becomes more mature. When 

 the generative organs are mature, fertilization of the ova occurs, the 

 ovaries and the testes disappear, and the uterus alone remains to store 

 the eggs. When the proglottis reaches this stage it is "ripe'* and 

 breaks off to pass out with the faeces (Fig. 158). Despite its con- 

 nection with the scolex, each proglottis must be regarded as an in- 

 dividual for it contains a full set of generative organs both male and 

 female. 



The generative organs are of the same type as is found generally 

 throughout the Platyhelminthes. There is a single opening for both 

 male and female organs. From the ootype there leads out a duct which 

 is called the uterus and is used for the storage of eggs, but it is 

 doubtful if it is homologous with the uterus of the Trematoda. 



The homologies of the various ducts of the genitalia of the Platy- 

 helminthes present great difficulties (Figs. 159, 160). While one or two, 

 the oviduct and the vas deferens for example, are quite clearly homo- 

 logous throughout, the homologies of others, particularly the accessory 

 organs such as uterus, bursa copulatrix, vagina, are very doubtful. 

 The "uterus" of the Trematoda is clearly the ductus communis of 



