The work which forms the subject of the present essay was carried out in the Cambridge Zoological Laboratory during the years 1895-7. The material on which my results are founded consisted of a collection of embryos and larvæ which had been obtained by Mr. Sedgwick and Dr. Willey during their visits to Faro in 1890-91, and I have to express my warmest thanks to Mr. Sedgwick for placing this valuable material at my disposal.
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E. W. MacBride (1898) studied this question.
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