MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 443 



Instead of adopting the.se familiar words, and giving to them a restricted meaning, and hav- 

 ing to deline whether the one or the other usage is to bo attached to them, I prefer to speaiv of 

 "unilateral pairs"' and "bilateral pairs," according as the two moieties arc situated on one side 

 of the polyp, or are on opposite sides of the polyp. There can possibly be no ambiguity as to the 

 character of the mesenterial pair indicated. 



The portion of the gastro-cuelomic cavity included within a unilateral pair of mesenteries is, 

 following the terminology proposed by Fowler (1885), known as an "Entocoele," while that 

 between any two such pairs is an "Exoccele." Further, the polyp can be divided into sextants 

 by six radii included within the primary entocfjeles, and the mesenteries or septa within each 

 sextant are spoken of as constituting a "System." 



A pair of directives occurs at each extremity of probably all sexually produced Madreporarian 

 and Actiniarian polyps, hut the regularity is often dei^arted from in asexually developed polyps 

 (p. -W:S). The vertical plane included within the two pairs of directives is known as the " Directiv^e 

 plane," and coincides with the axial or median plane of the polyp, as well as with the longer 

 diameter of the stomoda'um, and divides the polyp into synunetrical halves. Were the V 

 and VI pairs of mesenteries to become complete, it is clear that a plane passing between the two 

 pairs I and V would also divide the polyp ymmetrically into equal halves, and include the 

 shorter diameter of the stomodwum. Hence polyps at such a stage have two axes of perfect 

 sj'mmetry at right angles to each other. From the occurrence of directives, and of longer and 

 shorter diameters of the .stomodifium, perfect radial symmeti'v is not found in any of the present 

 species, and -rrliere noticed elsewhere is probably a result of asexual methods of reproduction — 

 not a fundamental characteristic. 



Among animals like coral polyps and anemones, exhibiting a certain degree of radial 

 symmetry, the terms dorsal and ventral and anterior and posterior, though adopted, have not 

 the same signilicance as in the higher animals, where one aspect of the body is altogether different 

 from the other. Moreover, the relationships, even as understood, ai'c not readily established in 

 adult polyps. To determine them it is necessary to select some morphological condition to which 

 the disposition of the organs can lie referred. The presence of directives enables a median plane 

 to be established, to which the organs on each side of the polyp are symmetrically related, right 

 and left, and such a mesenterial stage as that represented on page 441 also enal^les what may be 

 termed upper and lower borders to be established. The aspect of the polyp toward which the 

 faces bearing the longitudinal musculature of the two complete bilateral pairs of mesenteries I, II 

 are turned has been designated by Haddon (1889, p. 300) the " Sulcar," and the opposite the 

 "Sulcular." The terminology is based upon the fact that amongst Anthozoa where only one 

 gonidial groove (sulcus) is present (Alcyonaria, Zoanthese), the organ is on the aspect of the 

 polyp toward which the faces of the two pairs of mesenteries, referred to as bearing the vertical 

 musculature, are directed. As gonidial grooves, however, seem never to occur within the 

 Madreporaria, this character is of no assistance for purposes of orientation, and the sulcar and 

 sulcular relationships, as a rule, can onh- be determined from the order of development of the 

 first C3'cle of mesenteries. Where, in Zoantharian polyps, all the six pairs of protocnemes are 

 already complete, and either no gonidial grooves ai'e present or both are equally developed, there 

 is in ordinaiy cases no means of determining the sulcar and sulcular relationships. By most 

 writers on the Anthozoa the sulcar border is regarded as ventral and the sulcular as dorsal. 



Is it possible to determine an antero-posterior relationship in the polyps from the known 

 facts of their development, such as shall be at all comparable with that in the higher animals? 

 E. van Beneden (18!tl), from his study of the develoiiinent of the Cerianthid Arachnactis. and 

 E. B. Wilson (188'4), from his investigations on the mesenterial filaments of the Alcyonaria, follow 

 the suggestions of Sedgwick and Caldwell, and compare the gastro-c(jelomic chambers of the 

 Anthozoa with the ccelomic diverticula of the higher animals. On this theory the side of the 

 Cerianthid polyp on which the sulcus and directive mesenteries are situated is i-egarded as 

 anterior, while the side at which new mesenteries or segments are added is considered to be 

 posterior. From the arrangement of the mesenterial musculature, Carlgreii (1893) has shown 

 that the sulcus of Crrifintlnci is situated at tho op losite extremity of the polyp from its position 



