442 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



and other studpiits of the iunitomy of eorals indiaite that hi other genera and speeies the muscle 

 plaitiiig'8 likewise remain comparatively simple. 



The degree of complexity attained by the mesogktal foldings undoubtedly varies much v/ith 

 the state of expansion or retraction of tlie polyp, the jilaitings being often scarcely recognizable 

 in the former condition. Their character also changes in ditierent regions of the poh'p, and even 

 in ditierent parts of the same section (PI. IV, fig. 38). The mesentery of Orhicella, represented 

 on PI. IX, fig. 68, shows remarka})le ditlcrences in this respect, the peculiarities extending 

 even to the face liearing the oblique nuisculature. In the diagranuuatic and seniidiagrammatic 

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

 processes from the face of the mesoghca. 



From the figure on page 441 it is manifest that tli(> paired cliaracter 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 be considered as pairs. These are known as "Bilateral 

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

 their appearance in the larva. In l)ilateral mesenterial pairs the retractor muscle of each moiety 

 is on the face turned toward the sam(> aspect of the polyp. On the other hand, any two adjacent 

 mesenteries in whicli the longitudinal nutscles 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 oth(>rs, these may l)e known as ''Unilateral pairs." 



The two members of a unilateral jiair uuiy be cither unequal (one complete and one incom- 

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

 of nuu'h phylogenetic interest arc connected with these conditions (]). 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, rarely, if ever, simultaneously 

 by cycles, Init l)ilaterally from one aspect of the polyp to the other (p. 459). 



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

 the faces turned away from one anotlicr occurs only in the case of the two axial ])airs, which 

 by this means are distinguished as " I)irectives." Both from theii' origin and the disposition of 

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



An attempt has lately been made to I'estrict 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 thi^ opposite aide of the polyp, or whether thev are close together on the same 

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

 anything in the terms themselves to denote which should l)ear one special significance more than 

 another, and already they are employed in a directly opposite manner l)y 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'apptiraissent pas par couples, conmie dans la 

 periode precedente, c'e.st-a-dire, une d\in cote, une de Fautre cote de I'axe commissural de TActinie, 

 mais par ixdn-ft dans Ics interloges formces durant cette periode. II a etc explique (ju'une paire est 

 constituce par deux cloisons voisines dont les faisceaux de feuillets unilatcraux se font vis-ii-vis 

 (les faisceaux unilateraux des deux paii-s commensurales faisant, .seuls, exception) ct que chaque 

 pair forme une loge." Also van Bencden (1897, p. '21): "D'accord avec Faurot, j'estime (|u"il 

 y a lieu de reserver cxclusivement le mot paire pour designer deux clo.sons voisines delimitant 

 une loge; le mot couple pour denommer I'ensemble de deux sarcoseptes symetriques, siegeant Fun 

 a droite, Fautre a gauche du plan nu'^dian." 



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

 a couple would be the arrangement corre.sponding with what is here termed a hilatenil j>alr, and 

 pairvfWh what is here designated a Knilatemljxiir. 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 secpience to use the word 'pair,'" thus signifying directly the opposite of Faurot 

 and van Beneden. 



