A modular gas delivery system (100, 200, 300, 400) for delivering gas to a plurality of non-human animals simultaneously, the system comprising: an upstream module comprising a gas inlet; a downstream module comprising a gas outlet; and an intermediate module assembly (118, 418) comprising a first intermediate module, the first intermediate module comprising a first gas delivery outlet, and a second intermediate module, the second intermediate module comprising a second gas delivery outlet, wherein the intermediate module assembly (118, 418) is: engageable with, and disengageable from, the upstream module; and engageable with, and disengageable from, the downstream module, wherein the first intermediate module is engageable with, and disengageable from, the upstream module, wherein the second intermediate module is engageable with, and disengageable from, the upstream module, wherein the upstream module, the downstream module and the intermediate module assembly (118, 418) are configured to be assembled into a first system configuration and a second configuration, wherein in the first system configuration and in the second system configuration: the upstream module is upstream of, and engaged with, the intermediate module assembly (118, 418); the intermediate module assembly (118, 418) is upstream of, and engaged with, the downstream module; and a gas flow path is formed from the gas inlet of the upstream module to the gas outlet of the downstream module, wherein in the first system configuration: a first gas delivery flow path is formed from the gas inlet of the upstream module to the first gas delivery outlet of the first intermediate module, wherein in the second system configuration: a second gas delivery flow path is formed from the gas inlet of the upstream module to the second gas delivery outlet of the second intermediate module, and wherein the second system configuration is a parallel system configuration and, in the second system configuration, the first intermediate module and the second intermediate module are arranged in parallel such that the second gas delivery flow path does not extend through the first intermediate module. an upstream module comprising a gas inlet; a downstream module comprising a gas outlet; and an intermediate module assembly (118, 418) comprising a first intermediate module, the first intermediate module comprising a first gas delivery outlet, and a second intermediate module, the second intermediate module comprising a second gas delivery outlet, engageable with, and disengageable from, the upstream module; and engageable with, and disengageable from, the downstream module, wherein the first intermediate module is engageable with, and disengageable from, the upstream module, wherein the second intermediate module is engageable with, and disengageable from, the upstream module, wherein the upstream module, the downstream module and the intermediate module assembly (118, 418) are configured to be assembled into a first system configuration and a second configuration, wherein in the first system configuration and in the second system configuration: the upstream module is upstream of, and engaged with, the intermediate module assembly (118, 418); the intermediate module assembly (118, 418) is upstream of, and engaged with, the downstream module; and a gas flow path is formed from the gas inlet of the upstream module to the gas outlet of the downstream module, the upstream module is upstream of, and engaged with, the intermediate module assembly (118, 418); the intermediate module assembly (118, 418) is upstream of, and engaged with, the downstream module; and a gas flow path is formed from the gas inlet of the upstream module to the gas outlet of the downstream module, wherein in the first system configuration: a first gas delivery flow path is formed from the gas inlet of the upstream module to the first gas delivery outlet of the first intermediate module, wherein in the second system configuration: a second gas delivery flow path is formed from the gas inlet of the upstream module to the second gas delivery outlet of the second intermediate module, and wherein the second system configuration is a parallel system configuration and, in the second system configuration, the first intermediate module and the second intermediate module are arranged in parallel such that the second gas delivery flow path does not extend through the first intermediate module.