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The nervous system of the planktotrophic trochophore larva of Polygordius lacteus has been investigated using antibodies to serotonin (5-HT) and the neuropeptide FMRFamide. The apical ganglion contains three 5-HT-ir neurons, many FMRFamide-ir neurons and a tripartate 5-HT-ir and FMRFamide-ir neuropil. A lateral nerve extends from each side of the apical ganglion across the episphere and the ventral hyposphere, where the two nerves combine to form the paired ventral nerve cord. These nerves have both 5-HT-ir and FMRFamide-ir processes. Three circumferential nerves are associated with the ciliary bands: two prototroch and one metatroch nerve. All contain 5-HT-ir and FMRFamide-ir processes. An oral nerve plexus also contain both 5-HT-ir and FMRFamide-ir processes develops from the metatroch nerve, and an esophageal ring of FMRFamide-ir processes develops in later larval stages. In young stages the ventral ganglion contains two 5-HT-ir and two FMRFamide-ir perikarya; during development the ventral ganglion grows caudally and adds additional 5-HR-ir and FMRFamide-ir perikarya. These are the only perikarya that could be found along the lateral nerve and ventral nerve cord. The telotroch nerve develops from the ventral nerve cord. The 5-HT-ir and FMRFamide-ir part of the nervous system is strictly bilateral symmetric. and much of the system (i.e. apical ganglion, lateral nerves ventral nerve cord, dorsal nerve and oral plexus) is retained in the adult.

The cyclorhagid kinorhynch Echinoderes aquilonius Higgins & Kristensen, 1988 possesses a single pair of protonephridia located in segments 10 and 11. The protonephridia consist of: (1) three terminal cells T-1, T-2. T-3, each with two cilia; (2) a single non-ciliated canal cell; (3) a nephridiopore cell with many microvilli and a cuticular sieve plate. The protonephridia of Echinoderes are presumed to develop from the ectoderm near the area of the sieve plate on the eleventh segment, and are suspended in the dorso-lateral pseudocoelomic cavity where they are surrounded by a basal lamina. One of the terminal cells (T-1) secondarily penetrates the basal lamina of the tenth segment and a part of the cell attaches to the cuticle. The kinorhynch protonephridia are compared with the excretory organs of other Bilateria. expecially the ‘aschelminths’, and apomorphic characters of the kinorhynch protonephridia are defined.

The actinotroch larva of Phoronis muelleri has 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 of Branchiostoma are mesodermal. The protonephridium of the actinotroch is ectodermal.