The excretory organs of Branchiostoma are ectodermal in origin and comparable with the nephridia of annelids and platyhelminthes. The excretory organs of Branchiostoma are different from the kidney of vertebrates.
Protonephridia:
The protonephridia of Branchiostoma are simple, closed ciliated saclike and thin-walled ectodermal tubules. These are about 90 to 100 pairs arranged segmentally on dorso-lateral pharyngeal wall, one above each gill-slit on either side. Each protonephridium is a bent nephridial tube having a small upper posterior horizontal limb and a long lower anterior vertical limb. The vertical limb lies in the coelomic canal of the primary gill-bar and ends in a large group of solenocytes or terminates blindly.
The horizontal limb has numerous smaller groups of the solenocytes. On the ventral surface of the horizontal limb, opposite a secondary gill-bar, is an aperture called nephridiopore. It bears long cilia and opens into the atrium. A nephridium contains about 500 solenocytes. Each solenocyte has a round nucleated head leading into a thin tubule opening into the lumen of nephridium through independent aperture.
A long vibratile flagellum arises from the basal body of the cell (head) and extends into the tubule and project into the nephridium lumen. Tufts of solenocytes project into the dorsal longitudinal coelomic canals and are bathed in coelomic fluid. The nephridia are also supplied with small blood vessels.
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The nephridial tube opens by nephridopore into the atrium at the upper end of a gill-cleft lying against a secondary gill-bar. Thus, one end of the protonephridium lies in the coelom and the other end opens to the exterior (atrium). Protonephridia absorb nitrogenous waste from the blood sinuses and coelomic fluid by diffusion, and discharge it into the atrium.
Hatschek’s Nephridium:
Besides the paired series of protonephridia there is a single large nephridium of Hatschek above the velum, slightly to the left of the notochord. It is like a protonephridium with a bent tube having numerous tubules ending in solenocytes.
Its blind anterior end is just in front of Hatschek’s pit, while the posterior end opens into the prebranchial region of pharynx just behind the velum. The Hatschek’s nephridium is associated with a network of fine blood vessels and each solenocyte is surrounded by a small coelomic sac, it absorbs nitrogenous waste.
Brown Funnels:
There are two sac-like brown funnels which are situated dorsally upon the posterior end of pharynx, one on either side. The narrow anterior end of each funnel opens into the epibranchial or dorsal coelomic canal of its side, while the broad posterior end opens into the atrium. Most zoologists consider the funnels to be excretory, while some regard them to be receptor organs.
Renal Papillae:
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In the floor of the atrium are numerous small renal papillae, they are probably excretory.
The nephridia of Branchiostoma develop from ectoderm, so that in their development and structure they resemble the nephridia of polychaete Annelida, but more recent views discount these similarities and the resemblance is only a case of parallelism since the relationship of the two groups is very remote. The excretory organs of chordates are mesodermal kidneys which require a high arterial blood pressure for filtration, in Branchiostoma kidneys and heart are lacking, the low blood pressure is sufficient for excretion by protonephridia.