On Themisto libellula in Baffin Island Coastal Waters

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Abstract
The gross internal anatomy, food habits, local summer ecology and breeding cycle of Themisto libellula Mandt are studied in material from Baffin island coastal waters obtained in 1939 and 1940. The chitinous proventriculus projects backwards into the cavity of the midgut and forms a "gastric apparatus" which cuts off part of the wall of the anterior midgut from direct contact with the food. The gastric apparatus is strongly reminiscent of the peritrophic membrane in insects and the Onychophora. The food in the summer consists mainly of the smaller crustacean plankton; plant matter was also found in the gut. There is a high incidence of infection by a Gregarine parasite. The possible causes of the reduction in numbers of Themisto in the fjord waters in August are discussed; the reduction is tentatively referred to the agency of the outflowing surface current coupled with the habit of Themisto of seeking the surface waters. The breeding cycle is of the alternating, or two-phase type which has been recorded by the writer in other arctic planktonic forms. There is a breeding period in the early part of the year and perhaps another in the autumn.

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