442 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



and other students of the anatomy of corals indicate that in other genera and species the muscle 

 plaitings likewise remain comparatively simple. 



The degree of complexity attained by the mesogkeal foldings undoubtedly varies much with 

 tlic state of expansion or retraction of the polyp, the plaitings being often scarcely recognizable 

 in the former condition. Their character also changes in different regions of the polyp, and even 

 in different parts of the same section (PI. IV, rig. 38). The mesentery of Orbicella, represented 

 on PI. IX, rig. 68, shows remarkable differences in this respect, the peculiarities extending 

 even to the face bearing the oblique musculature. In the diagrammatic and semidiagrammatic 

 figures throughout the paper the retractor muscle is conventionally represented by simple 

 processes from the face of the mesoglcea. 



From the figure on page 441 it is manifest that the paired character of the mesenteries may 

 be regarded from two very different aspects. In the first place the corresponding mesenteries on 

 the two sides of the median axis may lie considered as pairs. These are known as "Bilateral 

 pairs," and so far as concerns the first six pairs, this is the manner in which the mesenteries make 

 their appearance in the larva. In bilateral mesenterial pairs the retractor muscle of each moiety 

 is on tlie face turned toward the same aspect of the polyp. On the other hand, any two adjacent 

 mesenteries in which the longitudinal muscles are on the faces turned toward each other- that is, 

 toward opposite aspects of the polyp — may also be conceived as pairs, and. in contradistinction to 

 the others, these may be known as "Unilateral pairs." 



The two members of a unilateral pair may be either unequal (one complete and one incom- 

 plete, as in tig. 82) or equal (both either complete or incomplete, as in fig. 81). Considerations 

 of much phylogenetic interest are connected with these conditions (p. 453). To distinguish a 

 unilateral pair constituted of two equal mesenteries I propose the term "Isocnemic," and for 

 a unilateral pair of two unequal mesenteries the term "Anisocnemic." In the majority of 

 corals and anemones the metacnemes arise as isocnemic pairs, randy, if ever, simultaneously 

 by cycles, but bilaterally from one aspect of the polyp to the other (p. 459). 



In most adult polyps the condition in which the longitudinal musculature of a pair is on 

 the faces turned away from one another occurs only in the case of the two axial pairs, which 

 by this means are distinguished as " Directives." Both from their origin and the disposition of 

 the musculature, the directives are bilateral pairs, and are always isocnemic. 



An attempt has lately been made to restrict the meaning of the nearly synonymous 

 words '"pair" and •couple." so as to imply whether the two moieties of a mesenterial pair 

 are situated on the. opposite side of the polyp, or whether they are close together on the same 

 side of the polyp, their retractor muscles being vis-a-vis. Unfortunately, there is scarcely 

 anything in the terms themselves to denote which should bear one special significance more than 

 another, and already they are employed in a directly opposite manner by different Anthozoan 

 writers. Thus Faurot (1895, p. 51), referring to the manner of appearance of the mesenteries 

 beyond the primary twelve, writes: " Ces cloisons n'apparaissent pas par couples, comme dans la 

 periode precedente, e'est-a-dire, nne d'un cote, une de I'autre cote de I'axe commissural de l'Actinie, 

 mais par paires dans les interloges forme*es durant cette periode. II a ete explique qu'une paireest 

 constitute par deux cloisons voisines dont les faisceaux de feuillets unilateraux se font vis-a-vis 

 (les faisceaux unilateraux des deux pairs commensurales faisant, seuls, exception) et que chaque 

 pair forme une loge." Also van Beneden (1897, p. -21): "D'accord avec Faurot, j'estime qu'il 

 y a lieu de reserver exclusivement le mot pain pour designer deux closons voisines delimitant 

 une loge; le mot c<>uj>1, pourdenommer I'ensemble de deux sareoseptes syinetriques. siegeant Fun 

 a droite, I'autre a gauche du plan median." 



There can be not the slightest doubt as to the sense in which these authors employ the terms; 

 a couplt would be the arrangement corresponding with what is here termed a bilateral pair, and 

 pair with what is here designated a unilateral pair. Yet Bourne, in the article "Anthozoa," in 

 Lankester's "Treatise on Zoology" (1900, p. 39), in a foot-note adds: "It is convenient when 

 speaking of the adult arrangement of the mesenteries to use the word •couple/ when of their 

 developmental sequence to use the word 'pair,'" thus signifying directly the opposite of Faurot 

 and van Beneden. 



