TAPE WORMS 255 



location of the genital pore and the character of the branching of the uterus are of 

 the greatest importance in differentiation. The sexually-mature proglottides may 

 either expel their ova, when these would be found in the faeces or, as is common, 

 they break off and pass out themselves in the faeces. Then they either expel the 

 eggs or may be eaten by some animal and in this way effect an entrance for their ova. 

 It is an important practical point that the fasces of a patient with T. solium or T. 

 saginatamay not show any ova, these passing out in the intact segments. The oval 

 operculated eggs of Dibothriocephalus latus, however, are constantly in the faeces. 

 The "hexacanth" or six-hooked embryo,-also called the onchosphere, is the essen- 

 tial part of the egg. The embryonic envelope is dissolved off in the alimentary 

 canal of the animal ingesting it, and the onchosphere bores its way through the gut 

 to later become encysted in various tissues. In some tape-worms a ciliated embryo 

 is liberated from the egg shell and swims about actively to enter some fish or other 

 animal. When the six-hooked embryo reaches its proper tissue, the hooklets are 

 discarded and a scolex similar to the parent one is developed. At this time we have 

 a bladder-like structure with the scolex inverted in it. This is termed the proscolex 

 stage. This little cyst with its scolex when ingested by another animal is digested, 

 and the scolex, establishing itself in the intestine, develops a series of segments. 

 The ciliated embryo of the D. latus does not form a cyst, but instead a worm-like 

 creature similar to the adult. This is termed a Plerocercoid. 



If the larval stage shows a single cyst and a single head, it is termed 

 Cysticercus; if multiple cysts but only one head to each cyst, Ccenurus; 

 while with multiple cysts and multiple heads in each cyst the term 

 Echinococcus is used. 



Where there is very little fluid in the cyst and the larva is of minute 

 size, as with the Hymenolepis, the term Cercocystis is employed. 



KEY TO CESTODE GENERA. 



I. Head with two elongated slit-like suckers Genital pores ventral Rosette 

 uterus. Dibothriocephalidcz. 



(A) Single set of genital organs in each segment. Dibothriocephalus. 



(B) Double set of genital organs in each segment. Diplogonoporus. 



(C) Immature fofms showing characteristics of Dibothriocephalidse (collective 

 group). Sparganum. 



II. Head with four cup-like suckers; genital pores lateral. T&niida. 



(A ) Uterus with median stem and a varying number of lateral branches. Tania. 



(B) Uterus without median stem and lateral branches. 



(1) Genital pores single. Rostellum with not more than two rows of hooks. 



(a) Suckers armed with numerous small hooklets. Fifteen to twenty 

 testicles in each segment. Davainea. 



(b) Suckers not armed. Three testicles in each segment. Hymenolepsis. 



(2) Genital pores double. Rostellum with four or five rows of hooks. 

 Dipylidium. 



