MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 441 



term which would distinguish these mesenteries, either singly or as :i whole, from the later 

 mesenteries. The twelve primary mesenteries constituting the first cycle were designated 

 •■ Protocnemes," and those appearing later " Metacnemes."" 



I'he appearance of a mesentery in transverse sections is rarely the same on both surfaces. 

 The cut surfaces of the longitudinal or retractor muscle fibers form a bead-like margin to the 

 mesoglcea, which is highly retractive and stains deeply (PI. XVIII, fig. 129). Usually the longitu- 

 dinal musculature on one face is stronger than the oblique musculature on the other, and as a rule 

 the mesogloea becomes folded or plaited to afford an increased area for the support of the former. 

 The oblique muscle fibers are on the opposite face, and in transverse sections are usually cut 

 obliquely, and the mesoglcea is rarely plaited for their support. For purposes of orientation, 

 when studying the internal anatomy of the polyp, the recognition of the retractor muscle libers 

 on one face or the other of a mesentery is of great importance. 



Sulculur, Dorsal. Anterior. 



E IE. 



Sulcar, Ventral, Posterior. 

 Fig. 3. 



Plan of the mesenteries at the close of the protocnemic stage. The stage occurs in the growth of probably all larval and bud polyps, 

 and is retained by most of the adult polyps of Madrepora and Pontes. The Roman numerals (I- VI) indicate the order in which the 

 mesenteries are found to appear in the larva of corals. The corresponding mesenteries on the two sides constitute bilateral pairs, 

 and the adjacent mesenteries on each side in which the retractor muscles are turned toward each other (II, V; I, VI) constitute 

 unilateral (anisocnemic) pair^; the members of the axial pair, III, III, are the sulcar or ventral directives, and the pair IV, IV the 

 sulcular or dorsal directives. The vertical plane included within the two pairs of directives is the directive plane, and also the axial 

 or median plane. 



The mesogkeal plaitings for the support of the longitudinal mesenterial musculature are 

 never greatly complicated in form in any of the coral species here studied. They may be quite 

 simple, as on PL IV, tig. 38, or the folds may become secondarily plaited as in fig. 130. In the 

 Actiniaria, on the other hand, the plaitings are often very finely subdivided in a dendriform 

 manner, stretching along nearly the whole vertical face of the mesentery, or restricted about the 

 middle to form a thick, broad, vertical band. The various figures given by Fowler, Bourne, 



"The substantive ^Kriiini" — a radius or spoke of awheel — was first employed in Anthozoan literature by 

 Haddon ami Shackelton (1891, p. 626) in the course < if their studies of the Zoanthete. In a foot-note with regard 

 to it they write as follows: " We have tried hard to discover a short term for a mesentery, which would readily 

 lend itself to combination with other words, hut without success. The objection to the word 'cneme' is that it 

 has reference to the appearance of a transverse section of an Actinian rather than to a mesentery as it actually 

 exists. As the investigation of the Zoantheae, at least, must principally be made by means of transverse sections, 

 this objection has not much weight." 



