Moi'ijliology of the Mwireporuiia. 



471 



remaining polyps are quite normal, having twelve tentacles — 

 six inner slightly larger than six outer. In the Hawaiian 

 Poeilloj)ore3 the proportion of modified to normal polyps is 

 about one in twelve, at)d the former are distributed irregularly 

 over any region of the colony. From an examination of 

 serial transverse sections there is no question that at their 

 origin in the disk the protrusions correspond in position with 

 the tentacles, only the direction of growth is inwards instead 

 of outwards. Where three downgrowths occur there are only 

 nine ordinary tentacles, where two downgrowths are present 

 there are ten external tentacles, and where one downgrowth 

 there are eleven tentacles ; the missing tentacles are replaced 

 by the modification. Unfortunately the diminution in the 

 number of tentacles was not noticed on the living expanded 

 polyps, indeed such an observation would be very difficult 

 to make considering the smallness of the polyps of Pc>Ci7/o^ora 

 and the closeness of the two circles of tentacles. 



Fijr. 



Transverse section, much enlarged, through an iiitrapolypal tentacle, 

 showing the vacuolated character of the endoderm and the axis of 

 mesoglcea. The nuclei are few in number, small and irregularly 

 shaped ; a few symbiotic zooxanthollse are represented. 



Histologically the downgrowths present certain marked 

 peculiarities. The relationships to the polypal layers indicate 

 that the cellular constituents are wholly endodermal, but the 

 cells differ much from those of the ordinary endoderm. As 

 shown in the various figures they are much longer, while 

 fig. 5 gives their api)earance when highly magnified. The 

 cells are much vacuolated, and the various stains employed 



