Anatomy of Gorgonid.-e. 113 



mesodermal cells; secondary water- vascular 

 canals; the muscles of the individual polyps. 

 Endodermal — The lining of the tentacular canal and of each 

 intermesenterial chamber; the lining of the 

 gastro-vascular space, or of interior of calicle; 

 the lining of the primary water-vascular canals. 



PLEXAURELLA DICHOTOMA.i 



ZoANTHODEME branching dichotomously, sometimes grow- 

 ing to a height of three or four feet. The branches grow ver- 

 tically and parallel with each other, giving a peculiarly stately 

 appearance to the larger zoanthodemes. 



Branches smooth and round, sometimes clavate but not so 

 decidedly so as in Briarcmn and traversed by an axis cylinder 

 of peculiar structure. They are more flexible than in Briareum 

 the color being often very nearly that of buckskin, varying from 

 gray to a light brown. The calicles are completely included, 

 their orifices being elongated so as to present the appearance 

 of a short slit rather than a pore as in most Gorgonias. These 

 slits look like small gashes cut haphazard all over the surface 

 of the zoanthodeme, their long axes lying at every possible 

 angle with each other. There is no swelling around the edges 

 of the calicular openings. These openings become oval and 

 finally round as the polyps protrude themselves, and reassume 

 their slit-like appearance as they close over the retreating 

 polyps. The calicles are so inserted into the branches that 

 their longitudinal axes are slightly inclined upwards, so that 

 the mouth of each calicle is somewhat higher than its base. 

 The individual calicle is more elongated than in Briareum. 



Polyps completely retractile. When fully expanded they 

 cover the tips, as well as the rest of the branches, as 

 closely as they can crowd. They are very sensitive to any 

 disturbance, the polyps over a large zoanthodeme quickly re- 



I Identified by the author at the Smithsonian Institution. The generic 

 name Plexaurella was proposed by Kolliker for a certain section of the old 

 genus Plexaura. 



