Following the researches on the food of young fishes, it was thought advisable to investigate the food of the planktonic invertebrates. Whenever possible, therefore, the food of the larger animals brought in by the tow-nets was noted, and also that of many of the smaller creatures down to some of the unicellular organisms, such as the tintinnids and those members of the Peridiniales which are holozoic.With the larger animals it was hoped to find which of them actually ate the young fishes, and by investigating their food in general ascertain how much they were actually competitors with the fishes.The present work is offered as a preliminary, and it is hoped to continue it, following it up especially with more experimental work on the living animals.The tow-nettings were examined fresh and the food noted. Sometimes a few hours elapsed between the taking of the sample and the examination, so that some of the catch was moribund in the jar. There is always the objection that the food might have been taken in the jar whilst the plankton was being brought in, and it is a matter of general observation that many medusæ and various pelagic animals will devour young fishes and almost anything living when crowded up with them in the hauls. In many of the organisms examined, however, the nature of the food was so consistent in various hauls and from various localities that it seems almost impossible to believe that it is merely accidental, and it is probable that what food one usually finds inside any planktonic animal is natural to it.
Molluscs are very important members of our marine fauna, and, since many of them are planktonic in their early stages, they contribute largely to the number of organisms available as food for plankton-eating animals. The present work includes the prosobranchs only, particularlythose from Plymouth which have been specially studied during the last few years. Several papers have already been published dealing with the Plymouth species (Lebour, 1931–6), and these are referred to in due course. The present paper brings together the above work and that of others and summarizes our knowledge of the larval prosobranchs of Britain, with a description of the eggs. Naturally there are still many gaps, but it is hoped that these may be filled in time. A preliminary paper on the subject has already appeared (Lebour, 1933f). Closely related foreign species are referred to for comparison with our British forms. Original notes on certain species and information on the echinospira larva ofCapulus ungaricusare given here for the first time. Figures of typical eggs and larvae are given.
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