UNSEGMENTED WORMS. 193 



by eating the raw flesh of the pig, in which the bladder-worm stage occurs. 

 The latter is obtained chiefly from beef and is more common in America. 

 Only by adequate cooking is the danger of infection removed. The Amer- 

 ican habit of eating beef rare contributes to the spread of the pest. Other 

 tape-worms infest, as their double host, the dog and the rabbit; the cat 

 and the mouse ; the shark and other fishes. The excretory system is a 

 pair of continuous lateral tubes with transverse connections in the various 

 proglottides (Fig. 92, ex}. The nervous system in the adult tape- worm 

 includes a rather complex series of loops containing nerve-cells, in the 

 scolex, with right and left lateral lines of nervous tissue running the 

 length of the strobila. There are numerous longitudinal, transverse 

 (circular), and dorso-ventral muscle fibres passing through the spongy 

 tissue of the worm. There is a well-developed external cuticle which 

 helps protect the animal from the action of the digestive juices of the 

 host. 



Phylum Nemathelminthes (Round- or Thread-worms). Nemathel- 

 minthes are elongated, cylindrical forms which taper at the ends. The 

 body is covered by a dense cuticle. Some are aquatic but most are para- 

 sitic at least during a part of their life. An alimentary tract is present 

 and has both a mouth and an anus. There is a coelom which is not 

 divided into chambers and contains a fluid without corpuscles. There is 

 no circulatory system other than this. There are no special respiratory 

 organs. The central nervous system consists of a ring around the oesoph- 

 agus. This contains some nerve cells. From this ring nerves arise at 

 various points and pass both forward and backward. The chief posterior 

 nerve is ventral, but there may be also dorsal and lateral ones. The sexes 

 are usually separate. Development is sometimes direct, sometimes in- 

 direct. The best-known representatives arc the round-worms (Ascaris), 

 different species of which are found in the intestine of man, of the pig, 

 and of the horse ; vinegar-" eels " ; trichina. 



Trichina is one of the most dangerous of the nematode parasites. The 

 sexually mature worm occurs in the intestine of the rat, the pig, man, or 

 other mammal. The young are retained by the mother in the uterus 

 until well developed. When born the young bore through the wall of 

 the intestine of the host and make their way to the muscles, where they 

 become encysted and cause degeneration of the muscle fibres and often 

 other acute symptoms of the disease known as trichinosis. The larvae 

 remain in their cysts indefinitely or until the death of their host. For 

 further development the flesh must be eaten. In the intestine of the new 

 host where the cyst is dissolved the adult condition is quickly reached, 

 reproduction takes place again, the embryos migrate into the muscles and 

 the new cycle is begun. We do not find here the non-sexual reproduction 

 that helped make the Liver-fluke so prolific, but the reproductive power 

 of Trichina is very great without this. It is estimated that an ounce of 

 " measly " pork may contain 80,000 cysts of Trichina, and that each 

 female produced from these embryos may contain at one time 1,000 or 

 more embryos. During her life she may produce ten times this number. 



