Part II. — Stichodactylince and Zonnthea'. 201 



out of numerous examples sectionized, I have been able to definitely ascertain 

 the macrocnemic arrangement in several. 



The mesenteries below the stomodseum are very short in transverse sections, 

 and extend but a little distance vertically ; two or three are continued for some 

 way below the others, but wliich are in relation to the directives could not 

 be determined. As the free edge of the mesentery leaves the stomodseum 

 it becomes deeply concave. Owing to this, and the shortness of the stomodseum, 

 the perfect mesenteries in transverse sections of some retracted polyps appear 

 free even before the stomodseum is reached, one half being still connected 

 with the concave disc, and the other with the column- wall, each with the fila- 

 mental tissue at its free termination. 



The endodermal ej^ithelium of the mesenteries resembles that of the body- 

 wall, and contains many zooxanthellse. 



In the upper region of the polyps, the mesenterial mesogloea as it leaves the 

 column-wall is much and irregularly thickened, and contains cell-islets, but 

 beyond this the layer is extremely thin. There is a distinct indication of a 

 parieto-basilar muscle on each side, but the longitudinal musculature is not 

 sufficiently developed to allow of a study of the paired arrangement of the 

 mesenteries being made. 



No reflected ectoderm occurs on any of the mesenteries. In transverse 

 sections around the termination of the stomodseum an appearance of such is 

 presented, but it is merely the Driisenstreif which here runs horizontally. The 

 tissue is never folded, as is usually the case, with the reflected ectoderm, while 

 granular cells and nematocysts are present in addition to the supporting cells. 

 Vertical sections also reveal a similar absence. 



For some little distance from their origin at the stomodseum, the filaments in 

 section have an irregular outline. They are simple throughout their length, 

 consisting only of the middle lobe or Driisenstreif. In the lowermost region 

 the mesenteries may divide at their free edge into three branches, each capped 

 by a filament which is cordate in transverse section. The latter is sharply 

 cut off from the rest of the mesentery, and stains much more deeply. The 

 mesenterial endoderm is usually thickened immediately behind the filament, 

 in some cases partly surrounding the filament; otherwise it differs in no important 

 respect from the remaining mesenterial epithelium. 



No gonads were present in any of the numerous examples sectionized. 



The form described above was trawled on several occasions from a depth of 

 10—14 fathoms on the Pedro Banks, Caribbean Sea, embedded in the superficial 

 tissues of some massive, dark-coloured sponges. Fi-om the number of sponges 

 trawled, each bearing the commensal Zoanthid, the species must be very abundant 



