Gemmule formation in sponges is helpful in
A. Parthenogenesis
B. Sexual reproduction
C. Only dissemination
D. Asexual reproduction

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Hint: Sponges are predominantly aquatic multicellular organisms which lack true tissue systems. They show no body symmetry and are mostly sessile organisms. Sponges bear numerous pores in the body. They have specialised cells that are responsible to perform specific functions. The reproduction process in sponges can be sexual or asexual types.

Complete answer:
Sponges are sessile creatures. They remain attached to the substratum upon maturity. They reproduce both by asexual or sexual methods. In the sexual methods, the gametes are formed by the mature sponges. As the sponges are hermaphrodite animals, they are capable of producing both the types of gametes simultaneously. The spermatozoa are formed later than the oocytes. They are ejected into the water and are carried along with the water current to fertilize the oocytes. The early development of the sponge larva occurs within the sponge and the free-swimming larvae is then released from the osculum.
When the sponges reproduce by the means of asexual reproduction, it occurs by the means of fragmentations, through budding or by the gemmules. In the process of fragmentation, a body part of the adult sponge breaks and it settles on a substratum and it gives rise to a complete organism. In the process of budding, an outgrowth is formed on the body surface of the parent sponge. This outgrowth is genetically identical to the parent and in due course of time, it gets detached from the parent’s body. In case, the outgrowth is not detached, it gives rise to a new colony.
In a special type of asexual reproduction, the sponges reproduce by the formation of gemmules. This method of reproduction is not very common but it is found only in the freshwater species of sponges. Gemmules are defined as mass of cells that are capable of developing into a new organism. They are produced asexually. The gemmules are small bud-like cells and are hard coated structures. The gemmules have an inner layer of amoebocyte cells that are surrounded by cells that are surrounded by a layer of collagen.
Gemmules are produced mostly in response to harsh environmental conditions. The internal buds of the gemmules are resistant to dehydration, freezing and are able to survive without oxygen supply. The gemmules are hardy structures and are able to withstand tough environmental challenges. The gemmules remain active during unfavourable conditions and upon availability of water and favourable conditions prevail, they begin to develop into a new sponge.

Hence, The correct answer is option (D).

Note: Sponges reproduce by various methods and in extreme unfavourable conditions, they reproduce by producing gemmules. Gemmules are ball-like small cells which are provided with a thick outer coating which prevents and protects from any external shocks like harsh environmental conditions. They are then liberated from the parent’s body and they settle down on the substratum. The gemmules upon receiving favourable conditions grow into a new sponge.