The excretory system in Amphioxus resembles the excretory system of invertebrates rather than higher chordates. Such type of excretory system is present in flatworms and polychaete annelids. The excretory organs are protonephridia. Some other organs and cells are also regarded to be excretory.
The protonephridia are ectodermal in origin. There are about 90-100 paired segmental nephridia situated on the lateral surface of the pharynx.
Each nephridium is roughly inverted ‘L’ shaped microscopic structure whose vertical limb is parallel to the primary gill-bar. The vertical limb lies in the coelomic canal of primary gill bar and ends blindly. The upper limb opens into the atrium through an aperture, the nephridiopore. The nephridiopore lies against the secondary gill-bar. From dorsal and anterior surface of nephridium numerous short branches arise. Each branch bears the opening of many fine tubules and each tubule ends into a solenocyte or flame cell. There are about 500 solenocytes in a single protonephridium each measuring about 50 µ in length. Each solenocyte bears a nucleus and atrium where as their narrow anterior ends open into the dorsal coelomic canals. These funnels are lined with columnar cells containing brown pigment supposed to be excretory products.
2. Nephridium of Hatschek
In addition to the paired nephridia, a single large nephridium of Hatschek, occurs above the roof of the pharynx laterally to the left dorsal blood-vessel. It resembles the paired nephridia in structure. It is a narrow tube which opens at its posterior end into the pharynx just behind the velum, and then passes anteriorly to end blindly just in front of Hatschek’s pit. It receives several solenocytes along its length and they are associated with a network of capillaries. The nephridia of Amphioxus are derived from ectodermal layer of the embryo as such they resemble those of non-chordates both developmental as well as structurally.
Excretory function is also attributed to the numerous renal papillae, found in the floor of the atrium. But this is not supported by experimental evidence.