58 



NATURAL HISTORY. 



of their development, the wall of the joint forming an enveloping sac. 

 Their farther history is thus described by Dr. T. Spencer Cobbold : — 



" The growth of the multitude of embryos within their interior causes 

 the proglottid sooner or later to burst, and the embryos thus become dis- 

 persed ; some are conveyed down drains and sewers, others are lodged by 

 the roadsides, in ditches and waste places, while great quantities are scat- 

 tered wide by wind or insects in every conceivable direction. Each 

 embryo within the egg is furnished with a special boring-apparatus, having 

 at its anterior end three pairs of hooks ; after a while, as it were by acci- 

 dent, some animal, a pig perhaps, coming in the way of these embryos, or 

 of the proglottids, swallows some of them along with matters taken as 

 food. The embryos, immediately on being transferred to the digestive 

 canal of the pig, escape out of the egg-shell and bore their way through 

 the living tissue of the animal to lodge themselves in the fatty parts of the 

 flesh, where they await their further destiny. The flesh of the animal thus 

 infested constitutes the so-called measly pork." Here they drop their 

 hooks and take on a form which for a long time was supposed to be a dis- 

 tinct animal. If, now, a man were to eat a bit of this infected pork in a 

 raw or partially cooked condition, these animals would be transferred to 

 his intestine, where they would fasten themselves and complete their devel- 

 opment into the form with which we started. 



The species of whose history we have thus given an abstract is Tamia 

 solium, which may reach a length of nine or ten feet. Tcenia mediocanellata, 

 a species without the hooks on the head, and which may reach a length of 

 twelve feet, passes the intermediate stage of its life in cattle, while Tcenia 

 echinococcns is scarcely a sixth of an inch long, and is further peculiar that 

 its adult condition is found in dogs, while the intermediate stage is passed 

 in man and other animals. The largest of the human tape-worms belongs 

 to the genus Bothriocejjhcdus, which has but two suckers on the head. It 

 sometimes reaches a length of thirty feet. What its history is is not yet 

 known, but certain facts render it probable that a part of its life is passed 

 in fishes. It is an interesting fact in the distribution of these forms that 

 Bothriocephalus is the common form parasitic in Russians, Poles, and 

 Swiss, while other Europeans and Americans are attacked by Tcenia. 

 Tcenia echinococcns is abundant in Iceland and not infrequently proves 

 fatal to its victims. 



Next in order come the round worms, or, better, thread-worms, the 

 latter name being an equivalent of the scientific name Nematoidea, which 

 means thread-like. Their habitat is as comprehensive as that of the flat 

 worms ; some live in moist earth, some in water, and some are parasites. 

 The terrestrial forms are far more abundant than the terrestrial flat 



