**Nephridia in origin is mesodermal.**

Nephridia are excretory organs found in various invertebrates, including annelids, mollusks, and arthropods. These organs play a vital role in eliminating metabolic waste products and maintaining osmotic balance in these organisms.

**Mesodermal Origin:**

Nephridia are derived from the mesoderm, one of the three germ layers formed during embryonic development. The mesoderm gives rise to various structures in the body, including muscles, connective tissues, and excretory organs like nephridia.

During embryogenesis, the mesoderm undergoes various differentiation processes, leading to the development of different cell types. In the case of nephridia, specific groups of mesodermal cells undergo further specialization to form the excretory structures.

**Formation of Nephridia:**

The development of nephridia involves a series of complex cellular and molecular events. It begins with the formation of a segmental mesodermal structure called the nephrotome. The nephrotome is formed from a localized population of mesodermal cells in each body segment.

Within the nephrotome, specialized cells called protonephridial cells arise. These cells differentiate further to form the functional units of nephridia. The protonephridia consist of a tubule lined with specialized cells that help in the filtration and excretion of waste products.

**Function of Nephridia:**

Nephridia function by filtering the body fluids and removing waste products, such as nitrogenous compounds and excess ions. The structure of nephridia varies among different invertebrate groups, but the basic excretory function remains the same.

In annelids, nephridia consist of a ciliated funnel-like structure called the nephrostome that collects fluids from the coelomic cavity. The collected fluid then passes through a tubule, where reabsorption and secretion processes take place. Finally, the filtered fluid is released through an opening called the nephridiopore.

In conclusion, nephridia have a mesodermal origin, as they are derived from specialized mesodermal cells during embryonic development. These excretory organs play a crucial role in maintaining the internal environment of various invertebrates, helping them eliminate waste products and maintain osmotic balance.

i think ans is b)ectodermal