506 MAYOR. 



PART II. ON THE POSTLARVAL DEVELOPMENT. 



1. Form of the Young Polyp. 



At the time of the flattening out of the young polyp soon after 

 fixation there is no evidence of tentacles. The six pairs of primary 

 mesenteries show clearly as grooves on the surface and when viewed 

 by transmitted light as dark lines. The oral aperture, as in the 

 planula, is oval. The young polyps were not studied in sections as it 

 was desired to preserve the skeletons. 



2. The Early Development of the Skeleton. 



The study of the skeleton has been confined to the skeletons of the 

 larvae which fixed themselves to the glass vessels in which they were 

 kept. In spite of numerous efforts to rear young polyps and to obtain 

 early stages in the development of the skeleton, only a few skeletons 

 suitable for studying the development of the primary ento-septa were 

 obtained. Two of these have been chosen for description. 



a. Description of Skeleton A. 



This skeleton (Plate 5, top figure) shows a thin calcareous layer, 

 the basal plate, covering the area of the glass to which the polyp has 

 attached itself. This layer is slightly thickened in the centre, appar- 

 ently an unusual condition; for a columella is not developed in the 

 older skeletons obtained. The six primary entosepta are arranged 

 radially, extending from near the periphery about half way toward the 

 center. The two entosepta on the left and the upper one on the right 

 show forking at their peripheral ends. The basal plate is thickened 

 near the entosepta in areas corresponding to the primary entocoels. 

 A thin outer ring which extended round the basal plate was largely 

 destroyed in the maceration, a small part of it is shown in the lower 

 part of the photograph. It surrounded the outer limit of the soft 

 tissues of the polyp. The opacity of the young polyp made it im- 

 possible to study the relations of the skeleton to the soft parts while 

 the animal was still alive. So an outline drawing was made of the 



