The internal buds, which are formed by the freshwater sponges are called as gemmules. These gemmules are tough and coated with a dormant cluster of embryonic cells.
Ex.Spongilla
These are formed during unfavourable conditions and help in perennation and dispersal.

Gemmules: Structure and Function

Gemmules are asexual reproductive structures found in certain invertebrates, specifically sponges of the phylum Porifera. These specialized cells serve a crucial function in the reproduction and survival of sponges. Gemmules are essentially clusters of cells surrounded by a protective layer, and they play a significant role in the sponge's ability to withstand unfavorable environmental conditions.

Structure of Gemmules
Gemmules are composed of a central mass of cells known as archaeocytes, which are undifferentiated cells capable of giving rise to various specialized cell types. These archaeocytes are surrounded by a protective layer called the shell or scleroprotein envelope. The shell consists of a protein matrix embedded with mineral particles, such as calcium carbonate or silica, providing strength and protection to the gemmule.

Formation of Gemmules
Gemmules are formed as a response to unfavorable environmental conditions, such as drought, freezing temperatures, or changes in water chemistry. When faced with these adverse conditions, sponges initiate the process of gemmulation to ensure their survival. The gemmules are typically formed within the mesohyl, a gelatinous matrix located between the sponge's outer and inner layers.

Function of Gemmules
The primary function of gemmules is to enable sponges to survive in unfavorable conditions by undergoing a period of dormancy until conditions improve. Here are the key functions of gemmules:

1. Resistant Structure: The protective shell or scleroprotein envelope surrounding the archaeocytes shields them from desiccation, freezing, and other environmental stresses. This structure allows gemmules to remain viable for extended periods, sometimes even years, until favorable conditions return.

2. Asexual Reproduction: Gemmules serve as a means of asexual reproduction for sponges. When conditions become favorable again, gemmules can germinate and give rise to new sponge individuals, thus contributing to the survival and population growth of the species.

3. Genetic Diversity: Gemmules can contain genetic material from multiple parent sponges, ensuring genetic diversity within the population. This diversity is essential for the adaptation and resilience of the species to changing environments.

4. Dispersal Mechanism: Gemmules can be dispersed by water currents or through attachment to other surfaces, allowing sponges to colonize new habitats and expand their range.

In conclusion, gemmules are specialized structures found in sponges that enable them to survive harsh environmental conditions. These asexual reproductive units provide protection, ensure genetic diversity, and serve as a means of dispersal, ultimately contributing to the resilience and success of sponge populations.