Ten stations, from high water to low water, were investigated along a mud-flat in the Bristol Channel.The mud was analysed for water content and grade. The values suggested, with other observations, that the stations were distributed over two different areas.Quantitative analysis of both macrofauna and microfauna revealed a dense mud population.The operative ecological factors, including the food cycle, are defined.

The plankton collected at a station in the Bristol Channel in 1936-7 is related to chemical and hydrographical factors.On account of the varying speeds of the tidal streams, the observations refer, not to a single point, but to a series of points along a length of the channel. Gradients of salinity, phosphate, and a possible complicated gradient of oxygen saturation, were determined, and, from the average monthly salinities, an annual cyclical movement of water deduced.

An account is given of the general structure of the body skeleton of the spatangid larva. A considerable change occurs in the skeleton as the larva increases in size, some parts being absorbed and new parts appearing. Of particular interest is the replacement of the anterior transverse rod by the ventral horizontal rod.The skeletons of larvae, young stages to late stages, of Echinocardium cordatum, E. flavescens, Spatangus purpureus and Brissopsis lyrifera are described. The larvae of Echinocardium and Spatangus are, in essentials, very similar; that of Brissopsis differs greatly from them. A striking difference is that in Brissopsis the ventral horizontal rod arises from the postoral arm and anterior to the anterior transverse rod, in the other two genera it arises from the body rod posterior to the anterior transverse rod.The replacement of the anterior transverse rod by a ventral horizontal rod is probably normal in clypeastroid larvae also.

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