2.38 THE STUDY OF ANIMAL LIFE CHAP. 



teans are the first animals to show an open food-canal, and 

 there is a body-cavity in the larvic at least. 



3rd Set of Worms. Nemathelminthes or Round 

 Worms. Class : Nematcda or Thread-worms .--The 

 " worms " of this class are cylindrical, like pieces of string 

 or thread. The body is covered by a firm glistening 

 cuticle, which is periodically moulted ; the body-wall is 

 muscular ; in most a simple food-canal extends from 

 end to end of the body ; the sexes are usually separate. 

 Many of the Nematodes live in damp earth and in rotten- 

 ness ; many are, during part of their life at least, parasitic 

 in animals or plants. We have already noticed how long 

 some of them " paste-eels," " vinegar-eels," etc. may 

 lie in a dried-up state without dying. The life -histories 

 are often full of vicissitudes ; thus the " ear-cockles ' 

 worm (Tylenchus tritici) passes from the earth into the 

 ears of w r heat, and many others make a similar change ; 

 the female of Sphcerularia bombi migrates from damp 

 earth into humble-bees, and there produces young which 

 find their way out ; others, e.g. some of the thread-w r orms 

 found in man (Oxyuris, Trichocephalus), pass from water 

 into their hosts ; others are transferred from one host to 

 another, as in the case of the Trichina with which pigs 

 are infected by eating rats, and men infected by eating 

 diseased pigs, or the small Filaria sanguinis hominis, 

 sometimes found in the blood of man, which passes its 

 youth in a mosquito. Somewhat different from the 

 other Nematodes are those of which the horse-hair worm 

 Gordius is a type. They are sometimes found inside 

 animals (water-insects, molluscs, fish, frog, etc.), at other 

 times they appear in great numbers in the pools, being, 

 according to popular superstition, vivified horse-hairs. 

 There is another allied class, Acanthocephala, including 

 a few genera, e.g. Echinorhynchus, of parasitic habit, 

 the young stages occurring in crustaceans and insects, 

 the adults in vertebrates. 



4th Series of Worms. The Annelids or Ringed Worms. 



-Class : Chsetopoda or Bristle-footed " worms." -In the 



earthworms (Lumbricus, etc.), in the freshwater worms 



(Nais, Tubifex, etc.), in the lobworms (Arenicola pisca- 



