200 ZOOLOGY 



At the posterior end of an old colony the proglottides (Figs. 93, 94) are filled with 

 the developing embryos, and on breaking away from the chain these brood cases 

 pass with the fecal matter from the intestine of the host. In this way it becomes 

 possible for the embryos to find the way into a new host. On being swallowed by 

 some suitable animal they break from their cysts, bore through the wall of the 

 digestive tract into the tissues. Here they grow, become encysted and at this 

 stage develop the head or scolex which' remains attached to the bladder-like cyst 

 (Pig. 93, A, B). Development stops at this point unless the flesh of this host is 

 eaten by some other animal. When this happens the bladder is thrown off, the 

 head becomes attached to the wall of the intestine of the new host, and the active 

 formation of the chain pf proglottides begins again. 



The more commo:fi tape-worms of man are Tmnia solium and Tmnia saginata. 

 The former is more common in Europe and is received into the system 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 American habit of eating beef 

 rare contributes to the spread of the pest. Other tapeworms infest, as their 

 two hosts, the dog and the rabbit; man and fish; 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 (Pig. 94, ex). The nervous system in the 

 adult tapeworm 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 strobUa. 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. 



Phylum Nemathelminthes (Round- or Thread-worms). — Nemathelminthes are 

 elongated, 'cylindrical forms which taper at the ends. The body is covered by 

 a dense cuticle. Many are aquatic, but some are parasitic 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 

 esophagus, from which 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 indirect. The best-known representatives are the round- worm s 

 (Ascaris), different species of which are found in the intestine of man, of the 

 pig, and of the horse; vinegar- "eels"; trichina; and numerous free-swimming 

 forms. 



Trichinella 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. Por 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, 



