<P> If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host; sometimes, infection is facultative--the parasite can survive and complete its life cycle without infecting that particular host species . Parasites sometimes infect hosts in which they cannot complete their life cycles; these are accidental hosts . </P> <P> A host in which parasites reproduce sexually is known as the definitive, final or primary host . In intermediate hosts, parasites either do not reproduce or do so asexually, but the parasite always develops to a new stage in this type of host . In some cases a parasite will infect a host, but not undergo any development, these hosts are known as paratenic or transport hosts . The paratenic host can be useful in raising the chance that the parasite will be transmitted to the definitive host . For example, the cat lungworm (Aelurostrongylus abstrusus) uses a slug or snail as an intermediate host; the first stage larva enters the mollusk and develops to the third stage larva, which is infectious to the definitive host--the cat . If a mouse eats the slug, the third stage larva will enter the mouse's tissues, but will not undergo any development . </P>

What is necessary for a nematode to be considered to have an indirect life cycle