AbstractThe actinotroch larva ofPhoronis muellerihas a pair of protonephridia located beneath the tentacle ring and draining the blastocoel; each protonephridium is composed of about 25 solenocytes and a nephroduct which opens in a nephropore on the ventral side of the metasome. The neck of the solenocytes consists of bars, mutually interconnected by a fenestration lamina. Inside the neck microvilli originate proximally in the proximal intrachoanal field and extend through the neck into the nephroduct. There is no canal cell. In cross section the nephroduct is composed of 5–7 monociliary cells, with the cilium protruding through a border of microvilli and extending into the nephroduct. The whole protonephridium is surrounded by a basal lamina. Comparisons of the actinotroch protonephridium with those of other groups have not revealed any convincing homologies. The protonephridia of the protostomians are all considered to be of ectodermal origin, while the cyrtopodocytes ofBranchiostomaare mesodermal. The protonephridium of the actinotroch is ectoderma