454 



WORMS. 



Dochmius duodenalis. a, Entire ; b, Head (enlarged) ; 

 c. Tail (enlarged). 



reach in this host a length of about one - twelfth of an inch, their complete 

 development only taking place after the swallowing of the infested Cyclops by a 

 fish. Another member of the same family is the Syngamus 

 trachealis, which owes its double name to the fact that the 

 males and females are found in pairs in the windpipes of 

 various birds. They sometimes occur in such numbers that 

 the inflammation set up by their blood-sucking suffocates their 

 host. The eggs appear to be brought up into the bird's mouth 

 by crowing, or by the choking cough that the presence of their 

 parent causes. 



They are then 



swallowed, and 



pass out through 



the alimentary 



canal. As soon as 



they have obtained 



sufficient damp- 

 ness and warmth, 



they develop in 



about a week's 



time into small 



thread-shaped em- 

 bryos, with a blunt head and pointed tail. These obtain an 



entrance into another, or the same bird's mouth with the food, 



and thence pass into the windpipe. 



Perhaps the most dangerous of all human internal parasite 



worms is Trichina spiralis. In the mature stage these 



creatures live in the intestines of mam- 

 mals and birds, where they propagate and 



gradually perish. The females are about 



one-eighth of an inch long, and twice 



the size of the males. In botli sexes 



the mouth lies at the front end of the 



body, which is its narrowest part ; the 



tail is stumpy, and in the male provided 



with a pair of short processes. The 



number of progeny produced by one 



female may amount to some thousands, 



and as soon as these are born they make their way into the 



blood-vessels of their host's intestine, where they are carried 

 by the circulation to some more distant part of the body, and 

 ultimately come to a stop in one of the muscles. Here by 

 feeding they grow in a few weeks to four times their original 

 size, and form between the muscular fibres a great cyst 

 or capsule, in the centre of which the worm lies coiled up in a spiral. It has 

 not been ascertained how long the creature can remain in this immature state, but 



head of Cucullanus 

 elejans (enlarged). 



HUMAN THREAD -WORM, 



Oxyuris vermicularis 

 (much enlarged). 



