1. Two Pseudophyllidea, Bothriocephalus scorpii (Müller) and Clestobothrium crassiceps (Rud.), have been obtained from Scophthalmus maximus (L.) and Merluccius merluccius (L.) respectively, from fishing grounds to the west of the British Isles.

2. The musculature of the scolex in both has been compared relative to the form and mode of function of that organ. All the major groups of muscles are present in both, but their arrangement is modified in C. crassiceps, due to the presence of hemispherical bothria, instead of open grooves, and a sphincter muscle around the bothrial opening.

3. The mode of attachment is discussed briefly in C. crassiceps, and in more detail in B. scorpii, where the structure of the scolex was found to be admirably suited to the form of the host's mucosa. The effect on the mucosa is slight, due to the absence of very powerful muscles and of other specialized adhesive structures.

4. The nervous system of the scolex in both species has been compared, and a fundamental similarity has been found. Modifications in C. crassiceps are associated with shortening of the scolex and the difference in form of the bothria.

5. The excretory system shows the same correlation with the form of the scolex.

The proportion of young honeybees which became infested with Acarapis woodi Rennie rose with an increased degree of infestation of the older bees and increased temperatures. But the most important influence controlling infestation seems to be the frequency with which young and old bees come into contact; this is probably determined by the foraging activity of the bees and the ratio of young bees to old bees; the percentage of infested bees in colonies diminished rapidly during nectar-flows and rose in periods of little nectar-flow.

The mortality of infested bees was only slightly greater than that of non-infested bees and became evident only after the bees had been in prolonged unfavourable circumstances. There were no obvious signs of disease in heavily infested colonies in summer. Colonies with more than about 30 or 40 % of infested bees seemed more likely to die in winter than non-infested colonies, but few colonies became so heavily infested even after a very poor season.