UNSEGMENTED WORMS !<;<) 



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

 fecal matter from the intestine. 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, in antici- 

 pation of the needs of the adult worm, the head or scolex which remains attached 

 to the bladder-like cyst (Fig. 93, A, B). Development stops at this point unless 

 the flesh of this host is eaten by some other animal. When this happens the blad- 

 der is thrown off, the head becomes attached to the wall of the intestine of the 

 carnivorous host, and the active formation of the chain of proglottides begins 

 again. 



The more common tape- worms of man are Tania solium and Tcenia 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 

 double host, 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 (Fig. 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 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 V: 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 ccelom 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. 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 indirect. The best- 

 known representatives are 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; 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 

 ther way to the muscles, where they become encysted and cause degeneration 

 of he 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, 



