The living blood-cells of tunicates have been studied by means of phase contrast microscopy. The species used were Ascidia atra Lesueur, Ecteinascidia turbinata, and Clavelina picta, all of which are common in the Bermuda region. Blood-cells in blood taken directly from the heart, or from the vessels, or elsewhere, particularly in the tunic of Ecteinascidia turbinata and Clavelina picta, were observed for periods ranging up to 24 h.

A new type of appendage is described for those cells which we have called vacuolate cells (‘Q cells’ of Fulton, ‘signet-ring’ and ‘colorless morula cell’ of George). Individual cells show from 1 to 5 appendages, which are beaded and actively undulate. Fragments from the appendages continue to undulate. Such fragments bear some resemblance to blood-platelets of higher vertebrates in their strongly adhesive properties.

In view of the morphological features, motility characteristics, and apparent functions, the terms lymphocyte and macrophage are considered appropriate for the finely granular and coarsely granular amoebocytes. No active swimming motions of the orange cells, nor of any other corpuscles were seen; but all of the types of cells, coloured and colourless, with the exception of the vacuolate cells, exhibit amoeboid progression.

No transformation of vacuolate into green cells was brought about in our experiments with varying strengths of acid, contrary to the earlier findings of Fulton. Many types of blood-cells in the tunicates have ‘fixed cell’ counterparts in the tissues. It is suggested that the large ‘bladder cells’ of the tunic of A. atra Lesueur may be such counterparts of the vacuolate cells of the blood.

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