1889.] NATURAL SCIENCES OF PHILADELPHIA. 107 



In the Sagartidse and Paractidse there is imbedded in the column 

 wall below the margin a sphincter muscle. In other forms, however, 

 such as the Bunodidse, which possess a circumscribed endodermal 



sphincter, that structure lies internal to the margin. If we assume 

 with the Hertwigs that the sphincter is a columnar structure its 

 situation in the Bunodidse would indicate that the margin is not the 

 boundary between the disc and column. 



Neither the margin nor the sphincter, however, can be considered 

 the morphological boundary of the disc, since both seem to vary 

 somewhat in position. The true criterion is to be found in the dif- 

 ference of histological structure presented by the disc and column 

 ectoderm. This layer in the disc possesses ectodermal muscle-cells 

 and a nerve-layer, which structures are absent in the column. The 

 tentacles resembling the disc in structure are to be considered out- 

 growths of that region, and passing outward from these one finds 

 that the characteristic structures of the disc gradually fade out and 

 are lost. It is impossible to say just where the change is completed, 

 but the region in which it occurs must be considered the boundary 

 between the disc and column. In Bunodes tceniatua and Aulactinia 

 ■stelloides I find that the sphincter muscles lie beneath the outer bor- 

 der of this indifferent region, and are consequently to be regarded as 

 columnar structures. 



In the Phyllacticke the sphincter muscle lies between the tenta- 

 cles and the fronds, and although the ectoderm in the region in which 

 it occurred, and in the area between the fronds or their representa- 

 tives and the margin was completely macerated away in the forms 

 studied, yet reasoning from the relations of the sphincter in other 

 forms we must conclude that the region between the margin and the 

 base of the tentacles is columnar, and that the fronds and outer dig- 

 ititbrm tentacles are column structures perhaps comparable to 

 acrorhagi, and cannot be considered homologous with tentacles. 

 Accordingly only one tentacle belongs to each intra-mesenterial 

 space, and the Phyllactidae must be referred to the sub-order Aetinime. 



Andres in the introduction to his Monograph, notes the fact that 

 the margin does not always mark the boundary between the disc 

 and the column. He proposes the term " collar " to denote the por- 

 tion of the column internal to the margin. Gosse's term " fosse " is 

 not applicable in all cases, as for instance in Condylactis where the 

 region does not form a depression, but is horizontal. 



