III. CESTODA 251 



The differences in the sexual apparatus influence the peculiarities of 

 the egg. In Bothriocepliakis it is large, has a tough shell with a lid, and 

 encloses a small egg cell with numerous yolk cells. The eggs of Tccnia are 

 small, with a layer of albumen and a delicate shell which is lost early. It 

 is replaced by a radially striped envelope secreted hj the embrj-o in a 

 somewhat advanced stage. It is in this condition that the eggs escape. 

 A further consequence is a difference in development. In most Bothrio- 

 cephalidae, as in the Trematoda, the egg must 

 enter the water for its further development. , ^^'| ' "^-- 



Here a ciliated oval larva escapes which con- 

 tains a six -hooked larva {oncospliara, fig. 229). 

 The ciliated envelope is temporary and is cast 

 off like the ciliated coat of the trematode larva. 

 The six-hooked larva enters a fish, becomes 

 encysted (pleiirocercoid) in muscles or viscera, 

 and changes directly into the head of a 

 Bothriocephalus. This on being taken with Fiq. 229. — Ciliated embryo 



food into the intestine of the proper host de- °f Bothriocephalus lalus en- 

 closing the six-hooked larva. 

 velops mto the adult. 



The history of the Tcenias differs considerably. The distinctions are 

 early recognizable, since the six-hooked larva lacks the ciliated coat but is 

 enclosed in its homologue, the envelope already alluded to. Since this can- 

 not open of itself, the young are set free by its digestion in the stomach. 

 Thus the eggs of Tania solium must pass into the stomach of the pig (em- 

 bryos in fffical matter get mixed in the food) and after being freed from 

 their shell in the stomach, the larvas with their six hooks bore through 

 the intestinal wall and migrate, using the blood-vessels in their course, 

 into the muscles, or more rarely other organs. Here they develop into 

 bladder worms {cysticerci), becoming oval and secreting a cyst to which 

 the pig adds an envelope of connective tissue. The cysticercus grows 

 by increase of cells, and by the infiltration of serous fluid, so that it 

 becomes distended into a delicate translucent vesicle. In T. solium the 

 microscopically small embryo can grow in three or four months to the size 

 of a bean or pea; in some species as large as a hen's egg. By invagination 

 the wall of the bladder produces the anlage of the scolex (fig. 230, r). 

 This is at first sac-lilve but soon increases in length, its growth being con- 

 fined by an envelope {d), so that it is bent. The scolex appears like a 

 white swelling through the wall of the bladder. 



At the apex of this blind sac arises the characteristic armature of the 

 scolex which makes it possible to say what tapeworm will come from the 

 cysticercus. Thus in T. solium there are four suckers and a crown of 



