THREAD- WORMS. 



681 



space to notice is of special interest, not only on account of its being one of 

 the most dangerous of parasitic worms to mankind, but also because it 

 spends the whole of its existence within the bodies of two hosts, carrying 

 the parasitic habit to a greater extreme than is met with elsewhere in the 

 group. This is the worm called Trichina spiralis, which gives rise to the 

 dreaded disease trichinosis. In the adult or sexually mature stage this 

 worm lives in the intestine of various carnivorous mammals, man being 

 amongst the number. But the minute young, which are born alive, instead 

 of making their way to the outer world in connection with excrementitious 

 matter, bore through the wall of their host's alimentary canal and entering 

 the blood-stream are carried by the circulation to various parts of the body, 

 ultimately coming to a standstill in the muscles. Here the little creatures 

 begin to grow and form, for themselves a protective capsule or cyst between 

 the fibres of the muscular tissue. Meat infested with these encysted worms 

 is said to be " trichinised " (Fig. 8, ). In this state the Trichinae remain 

 without attaining maturity until the meat in which they have taken up their 

 abode is eaten by another animal, whereupon the action of the digestive 

 juices of the new host, dissolving away the muscular fibres and the capsule, 

 set free the worms, which in their new surroundings quickly become adult, 

 and each female gives birth to thousands of new individuals. Man as a rule 

 contracts "trichinosis " through eating " trichinised " pork in an underdone 

 state. But, happily, since the discovery of the source of infection some fifty 

 years ago, and the introduction of stringent laws against the sale of unhealthy 

 pork, the disease has become far less common than in former times. 



Usually classified with the thread-worms, though differing from them in 

 the absence of mouth and alimentary canal, and in 

 other anatomical details, are the parasitic-worms, 

 known as Ecliinorhynchus, belonging to the group 

 Acanthocephali. These two names the former mean- 

 ing "spiny-beaked " and the latter "thorny-headed " 

 refer to the presence on the worm's head of a retrac- 

 tile proboscis, armed with rows of backwardly directed 

 hooks. The various species of Echinorhynchus are 

 internal parasites, the mature stages being found 

 in vertebrata of different kinds, and the immature 

 stages in certain invertebrated animals. For ex- 

 ample, a species known as Echinorhynchus gigas, which 

 infests the alimentary canal of the pig, passes the 

 earlier part of its life in the intestine of the larvse of 

 various kinds of chafers, such as the cock-chafer 

 (Melolontha vulgaris) or the rose-chafer (Cetonia 

 aurata). The migration from the one host to the 

 other is effected in the following manner : The 

 embryos of the worm, invested in a protective membrane, 

 after passing out of the pig's alimentary canal in con- 

 nection with excrementitious matter, are devoured by 

 the beetle-larvse along with their food. In the stomach 

 of the new host the investing membrane becomes 

 dissolved, and the worm, thereby set free, bores into 

 the intestinal walls of the larvae. These in turn are 



rooted up and swallowed by the pigs, and the young a, Echinorhynchus gigas. 

 Echinorhynchi thus become transferred again into the b, Sagitta Mpunctata. 



Fig. 9. 



