MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 441 



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

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

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



The 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 

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

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

 the mesogliea 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 mesogloea is rarely plaited for their support. For purposes of orientation, 

 when studying the internal anatoni}' of the polyp, the recognition of the retractor muscle fibers 

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



Sulcular, Dorsal, Anterior. 



IE IE. 



Siiloar. Ventral, Posterior. 

 Fig. 3. 



Plan of the mesenterie.s at the close of the protocnemic stage. The stage occurs in the growth of probably all lairal and bud polyps, 

 and i.s retained by most of the adult polyps of Madrcpora and Poritcs. 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) pairs: 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 mesogloeal plaitings for the support of the longitudinal mesenterial musculature are 

 never greatly complicated in form in any of the coral species here studied. They nia\' be (juite 

 simple, as on PI. 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 fineh' subdivided in a dendriform 

 manner, stretching along nejirly 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 siiljstantive "kvi}>i?/" — a radius or apoke of awheel — wa-s first employed in Anthozoan literature by 

 Haddon and Shackelton ( 1891, p. 626) in the course of their studie.s of the Zoantheie. 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, but 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 Zoanthew, at least, must principally be made by means of transverse sections, 

 this objection has not much weight." 



