. Elementary biology; an introduction to the science of life . Fig. 154. The jellyfish aurelia The mature medusa, a, reproduces sexually, the gametes being thrown into the water, where fertilization takes place. The egg develops into an individual having the general form of a hydra, fi, and attaches itself to a rock. The animal elongates and breaks up into a number of individuals by means of constrictions, so that it comes to resemble a pile of bowls. Each individual, when separated, turns over and swims away, changing into a medusa, a our coasts (see Figs. 154 and 155). The complete life hist
. Elementary biology; an introduction to the science of life . Fig. 154. The jellyfish aurelia The mature medusa, a, reproduces sexually, the gametes being thrown into the water, where fertilization takes place. The egg develops into an individual having the general form of a hydra, fi, and attaches itself to a rock. The animal elongates and breaks up into a number of individuals by means of constrictions, so that it comes to resemble a pile of bowls. Each individual, when separated, turns over and swims away, changing into a medusa, a our coasts (see Figs. 154 and 155). The complete life history includes both kinds of individuals, male and female, and two kinds of generations, sexual and asexual. Alternation of generations is also found in many parasitic animals, especially parasites that inhabit two or more different hosts at differ- ent stages in their development. Thus, the malarial parasite repro- duces in the blood of human beings by sporulation ; that is, by the formation of a large number of spores. But in the body of the mos- quito there are produced tiny protoplasmic structures that unite in pairs; that is, they conjugate. There is thus present a sexual method of reproduction and an asexual method, and these alternate regu- larly so long as the organism has the opportunity to pass from one host (man) to the other (mosquito) and back again (see pp. 403-407 and Fig. 209).
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Keywords: ., bookcentury1900, bookdecade1920, bookpublishe, booksubjectbiology
