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This work was begun by one of us in the spring of 1909, at the Zoological Station at Naples, when holding the Oxford biological scholarship.1 Owing to various reasons, the chief of which was ill-health, little more was done there than to collect material. It has been completed with assistance in reconstruction of sections and illustrations at Oxford in the department of Comparative Anatomy during the winter 1909-10. We must here express our gratitude to Prof. Bourne for the opportunity he has afforded us, and the encouragement he has given us to complete the work.
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