472 DISCOVERY REPORjTS 



cannot be discerned as distinct from the membrane. With larger embryos the sac pro- 

 jects as shown in Fig. 57 A, but is still in close contact with the frontal membrane. Two 

 sacs with large and apparently advanced embryos are free from the membrane except 

 for a distal attachment proximal to the operculum (Fig. 57 B), and an empty shrivelled 

 sac has a similar attachment (Fig. 57 C). In this last specimen the frontal membrane is 

 mainly convex, but shows a well-marked depression where the sac may be presumed 

 to have rested at an earlier stage. This depression is, however, not to be seen in the 

 specimen shown in Fig. 57 B. 



O.s 



J_ 







•2mm 



Fig. 57. A-C. Aetea anguina (Linnaeus). St. 11 87, 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), f.m. frontal membrane, o.s. 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 (191 3, 

 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 



