118 PROCEEDINGS OF THE ACADEMY OF [1889. 



to each ridge. These tubercles increase in size towards the margin 

 and several become grouped together upon elevations of the column 

 wall, giving rise to mulberry-like protuberances. Near the margin 

 the tubercles suddenly cease, forming, in contracted specimens, a 

 strong ridge bounding the dome-shaped area which forms in such 

 specimens the summit. This dome-shaped area belongs to the col- 

 umn, the animal being strongly contracted, and though without 

 tubercles shows clearly the continuation upwards upon it of the lon- 

 gitudinal furrows, and is, accordingly, marked by a series of radiating 

 ridges. 



In structure the tubercles of the column are solid, being eleva- 

 tions of the rnesoghea. This tissue throughout the column is very 

 thick, measuring on the average 1 mm. in thickness. It presents 

 numerous anastomosing canals filled with cells, as well as the delicate 

 canals, which have been described by Erdmann and others, very 

 distinctly. These canals are without doubt processes from the large 

 canals, and the structure of the zoanthan mesogloea may be com- 

 pared to that of a bone, such as a frog's femur, the anastomosing 

 canals being compared to the lacunas and the delicate canals to the 

 canaliculi. My preparations of M. tuberculata seem to show that the 

 lacunae arise from both the ectoderm and endoderm. In some of 

 my sections dee]) bays can be seen running from the endoderm up 

 into the mesoghea, and from their ends and sides numerous canali- 

 culi can be seen branching out. These bays can be found in various 

 stages of enclosure by the mesogloea, the cells which they contain 

 being in some cases continuous with the general endoderm, in other 

 cases almost separated from it and finally quite so. So too with the 

 ectoderm. The lacunas which have just been formed in this manner 

 are much larger than the majority of those scattered through the 



, mesogloea, these frequently consisting of only a few or even a 

 single cell, and further the newly-formed lacunae usually contain 

 zooxanthellas, whose presence is rare in the older ones. It would 

 seem as if many of the newly-formed lacunas become divided into 

 smaller portions which separate from each other, except by the del- 

 icate canaliculi, and at the same time undergo an alteration in the 



.histological structure of their cells, the zooxanthellas disappearing 

 and the cells becoming filled with refractive deeply-staining granules. 

 It seems not improbable that these altered cells are concerned in the 

 formation of the mesogloea, their granules being particles which will 

 later on be added to the matrix of the mesosrloea. 



