NEMATODE WOEMS. 59 



which is the result of the rapid division of the nucleus and the 

 formation of a number of cells in the ovum. The cell division 

 is equally distributed throughout the egg — that is, the segmen- 

 tation of the ovum is equal. Eventually from this segmented 

 ovum two cell layers are formed : these two layers become 

 invaginated or pressed in at one end, and there is thus formed 

 an embryo known as the gastruJu. From the two cell layers of 

 this gastrula are developed the body-wall and alimentary canal. 

 The young may encyst in an intermediate host, and in the cyst 

 stage are transferred to the permanent host : for instance, 

 Spiro'ptera ohtusa in the mouse is derived from cysts of this 

 species in the meal-worm — the larva of a beetle which lives in 

 flour, biscuits, and meal, and there eats the ova passed out by the 

 mouse. A large number of these round worms develop direct. 



Lastly, we may have changes taking place in the same animal, 

 the worms migrating from the intestines to the parenchymatous 

 tissues, as we shall observe happens in the case of Trichina 

 spiralis in pigs. 



There is one very important group of Nematodes to the farmer, 

 which develop and live in damp earth and j)lants. These cast 

 their skin whilst in the damp earth and are converted into 

 what are known as "Ehabditis,'' forms which have a double 

 enlargement of the oesophagus and teeth attached to the pharynx. 

 Some become sexually mature in the earth, and their offspring 

 again migrate and live as parasites ; others undergo several 

 ecdyses, becoming mature in the permanent host. The em- 

 bryo gradually elongates and becomes coiled up in the shell 

 (fig. 18, e). The free development may be in the form of a 

 metamorphosis which happens away from the parent form. 



The larvse, parasitic in animals and man, live in the paren- 

 chymatous organs of their host, either free or encysted in con- 

 nective tissue capsules ; whilst the adults live chiefly in the 

 alimentary canal. 



The embryos often have a curious boring tooth in front, or a 

 circle of spines. They moult their skin : frequent ecdyses or 



