Commensalism occurs in a few instances, but parasitism either external or internal is rare.
In the whole group of the Cryptonemiales the parasitism becomes more marked still.
As a matter of fact, however, the range of structural variation within the group is so wide, and the modifications due to parasitism and other causes are so profound, that it is almost impossible to frame a definition which shall be applicable to all the members of the class.
A few examples are appended: Some of the Uredineae also exhibit the peculiarity of the development of biologic forms within a single morphological species, sometimes termed specialization of parasitism; this will be dealt with later under the section Physiology.
For the complicated parasitism of isopods and Sacculinidae on the same hosts Giard and Bonnier (Bopyriens, 1887, p. 197) should be consulted.
Natural Protection Against Parasitism The living organism is a rich storehouse of the very materials from which parasites, both animal and vegetable, can best derive their nourishment.
The influence of parasitism has so profoundly influenced its structure that its affinities are obscured by the development of specialized and adaptive features.
The inquiline habit (" cuckoo-parasitism "), when one species makes use of the labour of another by invading the nest and laying her eggs there, is of frequent occurrence among Hymenoptera; and in some cases the larva of the intruder is not content with taking the store of food provided, but attacks and devours the larva of the host.
The fungus seems, on the other hand, to stimulate the algal cells to special development, for those in the lichen are larger than those in the free state, but this is not necessarily adverse to the idea of parasitism, for it is well known that an increase in the size of the cells of the host is often the result of the attacks of parasitic fungi.
(1904); "On Erysiphe graminis and its adaptative parasitism within the genus, Bromus," Ann.
In its systematized form, as a branch of botanical study, it is of recent date, and, as now understood, the subject first received special attention about 1850, when the nature of parasitism began to be intelligible; but many disjointed references to diseased conditions of plants had appeared long before this.
Parasitism, &c. Marshall Ward, On some Relations between Host and Parasite, &c., Proc. Roy.
In reality, however, the families designated Eproboscidea (Hippoboscidae, Braulidae, Nycteribiidae and .Streblidae), are not entitled to be considered as constituting either a suborder, or even a main division of the Cyclorrhapha; they are simply Cyclorrhapha much modified owing to parasitism, and in view of the closely similiar mode of reproduction in the tsetseflies the special designation Pupipara should be abandoned.
In epiphytic forms the rhizoids of the epiphyte often penetrate into the tissue of the host, and certain epiphytes are not known to occur excepting in connexion with a certain host; but to what extent, if any, there is a partial parasitism in these cases has not been ascertained.
The unexpected occurrence of these worms in pools and puddles, often in great numbers, has given rise to myths about showers of worms. They occasionally make their way into the human stomach with the drinking-water and are vomited; but this is a case of pseudo-parasitism - they are no true parasite of man.
A curious effect of parasitism, well illustrated in crabs, though not confined to them, has been expounded by Professor Giard, namely, that it tends to obliterate the secondary sexual characters.