310 ZOOLOGY sect. 



Many of the Nematoda have a curious and complex life- 

 history : a few examples will be selected for description. 



Rhdbdonema nigrovenosum lives, in the sexual condition, in the 

 lungs of Frogs and Toads : it is remarkable among members of 

 the class in being hermaphrodite. The eggs are laid and the 

 embryos pass from the lungs into the enteric canal of the host, are 

 expelled with its faeces, and develop in water into a sexual 

 Nematode, called the Bhabditis- form, in which the sexes are 

 separate : in this the fertilised eggs develop in the body of the 

 female, and, when fully formed, make their way through the wall 

 of the uterus and proceed to devour, the whole of the maternal 

 tissues, leaving nothing but the cuticle. Being set free, they live 

 in mud until they succeed in gaining access to a frog's mouth, 

 when they pass into the lung, develop hermaphrodite reproductive 

 organs, and so re-commence the cycle. It will be seen that we 

 have here a peculiar form of alternation of generations, distinguished 

 not by the alternation of a sexual with an asexual form (meta- 

 genesis) as in Hydrozoa, but by the alternation of a hermaphrodite 

 with a dioecious form. This type of alternation of generations is 

 distinguished as heterogeny. 

 ^ One of the most terrible parasites of man is Trichinella spiralis 

 / (Fig. 249), a minute worm, the male (C) a little over 1 mm. (^ in.) 

 in length, the female (B) about 3 mm. (\ in.). In the adult or 

 sexual condition it lives in the intestine of Man, the Pig, and 

 other Mammals. 



The adult females, which are viviparous, leave the cavity of the 

 intestine and bore into its wall, usually reaching the interior of 

 one of the lacteal vessels of the lymphatic .system. Here 

 they deposit their young (B, e) to the number of as many as a 

 thousand or more at a time. These are carried passively in the 

 stream of lymph, perhaps ultimately in the blood-stream, and thus 

 distributed throughout the body. Eventually they travel into the 

 system of voluntary muscles, such as those of the • limbs, back, 

 tongue, etc. Each worm then penetrates the sarcolemma of a 

 muscle-fibre and coils itself up in the muscle substance (A); a 

 spindle-shaped cyst (cy.) is formed round it, and the muscle 

 undergoes more or less degeneration. This process gives rise to 

 various morbid symptoms in the host, but, after some months the 

 cysts become calcified, and the danger to the infected individual is 

 over. The flesh of a " trichinised " human subject has been 

 estimated to contain 100,000,000 encysted worms, and that of an 

 infected pig 85,000 to the ounce. In order that further develop- 

 ment of the encysted and sexless Trichina; should take place, it is 

 necessary for the infected flesh of the host to be eaten by another 

 animal in which the worm is capable of living, e.g. that of Man 

 by a Pig or Rat, or that of a Pig by Man. When this is done the 

 cysts are dissolved by the digestive juices, the worms escape, 



