200 



LOWER INVERTEBRATES. 



Fig. 183. - 



■ Twnia echinococcits from pig 

 aud dog. 



in front, and the steady enlargement of those once formed, until the hindmost ones, 

 being fully matured, drop off the chain. 



Taenia mediocanriMlata is distinguished from solium by the want of hooks on the 

 head, and by the fact that the broadest proglottids are those half mature at the middle 

 of the ribbon. The Cysticercus stage is passed in the flesh of cattle. 



In Tcenia echinococcus the history is reversed, and offers many extraordinary pecu- 

 liarities besides : reversed because the larval stage infests man and other animals, while 



the adult is found in one of man's domestic animals, 

 the dog ; peculiar chiefly because one larva produces 

 several adults, and also because the larva differs very 

 much when found in man from its form otherwise. 

 The larva in the pig bears the name of echinococcus ; 

 it is a large round vesicle usually the size of a walnut, 

 but sometimes growing to be as big as an orange. 

 Usually it is found in the liver, sometimes in the 

 lungs, and even in other positions. The thin-walled 

 bag has several ingrowths, as shown at A in the 

 accomi^anying wood-cut, each ingrowth is an in- 

 vaginated he.id, and becomes a distinct adult indi- 

 ^-idual. These are everted and broken off, and are 

 found in the intestine of the dog shortly after it has eaten the infested flesh of a pig. 

 The crown of numerous small hooks around the head enables the young worm to 

 anchor itself. It forms only a few proglottids at a time, Fig. B, so that the adult is 

 not more than four millimeters long, the terminal joint soon matures, breaks off, and is 

 replaced. In man, however, the larva is still moie conlplicated, for from the main 

 vesicle grow out secondnr\' sacks, and from the walls of these latter the heads are sus- 

 pended ; the heads tlius lie in distinct capsules outside the main vesicle. The illustra- 

 tion, copied from Leuckart, shows the edge of the central 

 sack with one " Brutkapsel " projecting from it. The 

 Icelanders are very extensively afflicted by this para- 

 site, and the fact is attributed to their want of cleanli- 

 ness and the number of dogs that they keep around 

 them. The dogs scatter about the proglottids with 

 their dung, leaving the eggs directly or indirectly upon 

 the plants which the Icelanders eat; for they gathei' 

 for food certain mosses, sorrel, dandelion, and so forth, 

 from the midst of the plains, in which live flocks of fig. is*.— rcmja ecAinococcus, a single 



^ "Brutkapsel," from man. 



sheep guarded by dogs. The vesicles are found in all 



parts of the body, for when the larvas are set free in the intestine of man they wander 

 about everywhere, until they finally come to rest, and change to the many-headed 

 vesicles. As the vesicles, or hydatids, as they are often called, enlarge, they produce 

 very serious disturbances, and are often fatal. 



Besides the echinococcus, dogs are infested by Tcmiia serrata, which lives as the 

 Gxjsticercus pisiformis in the peritonaeum of rabbits and hares, and also by T. cucu- 

 merince. "Some years ago," says van Beneden the elder, "while making a post 

 mortem examination, at the Museum of Paris, of some young dogs which I had pre- 

 viously infected with TcBnia serrata at Louvain, there were found by the side of these 

 some Tcenia cucumerince. These dogs had taken nothing but milk and Cysticerci ! 



