Symbiont

As symbiont is defined as the smaller of the two species involved in a symbiosis. The spouse with the larger body is also called the host. As endosymbiont symbionts are called that - intracellularly or extracellularly - live inside their host.

For example, numerous families of bacteria as symbionts in vertebrates are known to occur especially on the skin, mucosa and in the intestinal lumen of their host; as an adult has a total of about 100 billion endosymbiotic bacteria with a total weight of about 2 kg.