. A manual of zoology. Zoology. 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 eg
. A manual of zoology. Zoology. 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 grow
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
