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Isogamy is found in
A. Hydra
B. Monocystis
C. Plasmodium
D. Planaria

Last updated date: 16th May 2024
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Hint:It is a parasite and in full growth stage it is called trophozoite. The body of this parasite is elongated and fusiform in shape.

Complete Answer:
Isogamy is a type of sexual reproduction involving gametes of identical morphology (identical shape and size), which usually varies only in allele expression in one or more mating regions. Since both gametes look the same, they cannot be categorized as "female" or "male." Instead, animals undergoing isogamy are stated to have separate mating forms, most generally referred to as "−" and "+" strains, although there are more than two mating forms in some Basidiomycota species. In both cases, fertilisation happens as gametes with two different forms with mating unite to form a zygote. Many unicellular species are isogamous.
Hydra reproduces asexually in a mechanism known as budding. For Hydra, this is the most common form of reproduction and occurs under suitable environmental conditions. During budding, a small bud forms near the basal part of the Hydra parent by regular mitotic division of the epidermal interstitial cells.
Monocystis is a parasite mostly found in the coelom and seminal vesicle of earthworms. Fusing gametes can differ physiologically. The isogamous gametes unite to form a zygote that grows into a sporoblast, a sporocyst, a trophozoite, and eventually a young trophozoite.
Plasmodium reproduces by both asexual and sexual means. It is a digenetic animal which life cycle is completed in two hosts. The primary host is female anopheles of the mosquito, where sexual reproduction occurs by amalgamation of male and female gametocytes.
In the case of asexual reproduction, the planar detaches the tail end and each half regenerates the missing parts.

Thus, the correct answer is option B. i.e., Monocystis.

Note:There are a variety of forms of isogamy. In some types both gametes have flagella and thus motile. This type of isogamy exists, for example, in algae including some but not all Chlamydomonas types.