MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 44)5 



Instead of adopting these familiar words, and giving to them a restricted meaning, and hav- 

 ing t<> define whether the one or the other usage is to be attached to them, I prefer to speak of 

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

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

 character of the mesenterial pair indicated. 



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

 following the terminology proposed by Fowler (1885), known as an "Entoccele," 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 entocoeles, and the mesenteries ,>r septa within each 

 sextant are spoken of as constituting a "System." 



A pair of directives occurs at each exl remit j of probably all sexually produced Madreporarian 

 and Actiniarian polyps, but the regularity is often departed from in asexuallj developed polyps 

 (p. 1 18). The vertical plane included within the two pairs of directives is known as the " Directive 

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

 diameter of the stomodseum, and divides the polyp into symmetrical halves. Were the V 

 and V] 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 stomodseum. Hence polyps at such a stage have two axes of perfect 

 symmetry at right angles to each other. From the occurrence of directives, and of longer and 

 shorter diameters of the stomodseum, perfect radial symmetry is not found in any of the present 

 species, and here 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 sa significance as in the higher animals, where one aspect of the body is altogether different 



from the other. Moreover, the relationships, even as understood, are 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 be 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 enables 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, Zoantheaa), 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 

 Madreporai'ia, this character is of no assistance for purposes of orientation, and the sulcar and 

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

 first cycle of mesenteries. Where, in Zoantharian polyps, all the six pairs of protocnemes are 

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

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

 w riter- 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 -hall beat all comparable with that in the higher animals? 



E. van Beneden (1891), from his study of the development of the Cerianthid Arachnactis, and 



F. B. Wilson (I>s4). from his investigation- on the mesenterial filaments of the Alcyonaria. follow 

 the suggestions of Sedgwick and Caldwell, ami compare the gastro-ccelomic chambers of the 

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

 Cerianthid polyp on which the sulcus and directive mesenteries are situated is regarded 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, Carlgren (1893) has shown 

 that the sulcus of Cerianthw is situated it tic op i isite extremity of the polyp from its position 



