. J_ 0 •2mm Fig. 57. A-C. Aetea anguina (Linnaeus). St. 1187, Tristan da Cunha. Distal ends of zooecia showing sup- posed ovisacs. Sculpture omitted. A. Ovisac closely appressed to frontal membrane. B. Ovisac free from frontal membrane, embryo more advanced than in A. C. Ovisac empty and shrivelled. em. embryo (stippled), frontal membrane, ovisac. I have seen the sacs in fifty-nine zooecia, and their constantly symmetrical position and absence from other parts of the zooecium give a very strong impression that they are part of the Polyzoan, and not the attached egg-capsule of ano
. J_ 0 •2mm Fig. 57. A-C. Aetea anguina (Linnaeus). St. 1187, Tristan da Cunha. Distal ends of zooecia showing sup- posed ovisacs. Sculpture omitted. A. Ovisac closely appressed to frontal membrane. B. Ovisac free from frontal membrane, embryo more advanced than in A. C. Ovisac empty and shrivelled. em. embryo (stippled), frontal membrane, ovisac. I have seen the sacs in fifty-nine zooecia, and their constantly symmetrical position and absence from other parts of the zooecium give a very strong impression that they are part of the Polyzoan, and not the attached egg-capsule of another animal. On the other hand it is difficult to reconcile these observations with those of Waters (1913, p. 463), Osburn (1912, p. 220) and Marcus (1937, p. 27) of distal embryo-sacs in this species. The older, partly detached, sacs somewhat resemble those of Osburn and Marcus, but the attachment in my material is certainly proximal to the orifice. In Waters's figure the ovisac is shown as a spherical vesicle attached to the calcareous wall and unconnected with the aperture. In view of the resemblance between the ovisacs of Aetea and those of such Ctenostomes as Nolella (see Harmer, 1926, p. 193) it might be suggested that their position is indeterminate were it not found to be so constant in any one batch of material. The frontal ovisacs appear to be the "membranous bags" described by Robertson (1905, p. 245) as "ooecia". The apparently asymmetrical position of the sac in her
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