(1.) It is shown that the animal studied belongs to the genus Pomatoceros. Its specific status is uncertain. (2.) The species is widely distributed throughout New Zealand and has also been reported from South Africa. (3.) The anatomy and some of the more interesting aspects of histology are described in detail, comparison being made with other Serpulids. (4.) The longitudinal muscles are well developed and the circular muscles much reduced, an adaption to the tubicolous habit. (5.) One pair only of nephridia is present in the thorax, opening internally by large ciliated coelomostomes into the peristomial coelom and externally by a common pore at the anterior dorsal end of the body. Excretory products are probably extracted from the blood in the form of guanine. (6.) The nervous system consists of a brain, formed from two pairs of united ganglia, situated in the prostomium and united to two sub-oesophageal ganglia in the peristamial segment by dorsal and ventral connectives on each side. The two ventral nerve cords are widely separated and the giant nerve fibres are particularly well developed. (7.) The blood system consists of a gut sinus, connected to a ventral vessel by paired ring vessels in each segment. From the ring vessels branches supplying the various organs of each segment arise. The capillaries of these vessels end blindly. Movement of the blood is effected by rhythmic peristaltic contractions of the walls of the vessels. Details of the circulation are described. When the animal retracts within the tube the blood circulation stops. This reversible stoppage of the blood is brought about by the accumulation of carbonic acid. The course of the respiratory currents within the tube is described. (8.) The ciliary feeding mechanism of the crown is described, the food consisting of finely divided plankton and detritus. (9.) The form of the tube is extremely variable. It is shown to be composed of a glyco-protein of a mucoid nature in which crystals of calcium carbonate in the form of aragonite are deposited. It is formed as a discontinuous secretion from gland cells of the collar region of the peristomial segment. The evidence so far collected points to the sea-water as the source of calcium. (10.) The development from the egg to a fully formed trochosphere has been followed. The egg is small with little yolk and development is rapid. (11.) A large percentage of the worms is infected by a gregarine parasite and large numbers of a commensal ciliate, Trichodina sp. are present. (12.) Experimentally Pamatoceros is found to tolerate a wide variation of temperature and salinity, and is shown to tolerate exposure and coverage by sand to a large extent. (13.) The habitat of Pamatoceros coeruleus is described in detail and a detailed analysis of the community at Taylor's Mistake, Banks Peninsula, to which it belongs, has been made. The relationship of a number of different species of plants and animals to tidal level and exposure to air is discussed, comparison being made with other surveys. Critical levels for the different species have been detected. Pomatoceros coeruleus is shown to be a dominant organism in the Chamaesipho-Mytilus planulatus Association of the littoral rocky shore. The general zonation of the plants and animals on the shore is discussed in relation to tidal level and exposure to wave action. A comparison is made with other surveys carried out in Australia, South Africa, North America and Great Britain. A fundamental basic zonation of typical indicator animal species, common to the temperate regions of the world is recognized. This basic scheme is, a Littorina zone, occupying the highest level on the shore followed by a Barnaole zone below with a Laminaria or Kelp zone occupying the sub-littoral fringe.