Development within definitive host

In spite of the vertebrate intestine being a biologically hostile environment for parasites, cestodes have been very successful in developing a wide range of morphological, physiological and biochemical adaptations which enable them to become established and to reproduce there. 

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Tapeworm. A parasitic intestinal cestode worm having a flattened, band-like form. The eggs of tapeworms are ingested by the intermediate host, they produce the larval stage in tissues. When the flesh of intermediate host is eaten, the larvae develop within the alimentary canal of the definitive host into adult tapeworms. 

With the exception of a few species with progenetic plerocercoids (e.g. Schistocephalus) it is only within the definitive host that cestode tissue develops in a strobilar direction, and it is self-evident that strobilar differentiation must be induced by factors present in the intestinal environment. Clearly, any number of known parameters of the intestine (Fig. 3.1, p. 37)... 

Brian also calls attention to the fact that certain parasitic microorganisms within plant tissues can produce antibiotics that are toxic to their hosts. Among these are lyco-marasmin, fusarinic acid, alternaric acid and patulin. There appears to be considerable doubt as to the connection between these antibiotics and the development of disease symptoms in the host plants. Definite conclusions must await further research. 

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