Vol. 88, No. 2 April, 1945 THE BIOLOGICAL BULLETIN PUBLISHED BY THE MARINE BIOLOGICAL LABORATORY THE SIZE OF THE MESH OPENINGS IN MUCOUS FEEDING NET5 OF MARINE ANIMALS G. E. MAcGINITIE William G. Kcrckhoff Marine Laboratory, California Institute of Technology, Corona Del Mar, California INTRODUCTION In a former article ( MacGinitie, 1937) the statement was made that "the slime net (Urechis) will entrap the smallest particles which are visible with the aid of an oil immersion lens." Experiments have just been completed which have determined much more accurately the size of the openings through which sea water is passed to strain from it the food ingested by animals using such nets. The results of these experiments have more than justified the statement quoted. THE FEEDING NETS The mucous net of the echiuroid worm Urechis caupu (Fisher and MacGinitie, 1928; MacGinitie, 1935) is admirably suited to tests of the kind herein reported. In the mud flats of the west coast Urechis builds a U-shaped tunnel with two en-trances eighteen to thirty-six inches apart, depending on the size and age of the animal, and with the lowest part of the tunnel from ten to eighteen inches beneath the surface. The openings of the burrows are somewhat constricted, and the ani-mals, which live singly in a burrow, keep a current of water flowing through the burrow along the body by means of a more or less rhythmic peristalsis. The slime tube for collecting food is secreted by a ring of mucous glands situ-ated about one-sixth of the length of the body posterior to the proboscis. When the worm secretes a mucous tube, this ring of glands is pressed tightly against the periphery of the burrow to cause the mucus at the beginning of secretion to ad-here to the sides of the burrow. As the animal backs up in its burrow more mucus is spun until the tube becomes four to six inches long. Then the spinning ceases, but the tube is allowed to remain attached to the secreting ring. This leaves the animal with its anterior end within a sack of mucus which is firmly attached to the "neck" as though by a drawstring ; and the distal end of the mucous sack is firmly attached to the sides of the burrow. Thus all water flowing through the burrow must pass in at the open distal end and pass through the walls of the mucous sack before it can flow along the outside of the animal on its way through the burrow. As the water passes through this mucous net all microscopic and, as will be seen, 107
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