432 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



figure of the larva, at the stage where twelve tentacles are present, should be compared with the 

 figures of the tentacles in the adult polyps of Madrepora and Porites, on l'ls. I and IV. It is seen 

 how verv closely they agree in the relative sizes of the tentacles, and especially in the promi- 

 nence (if one of the axial tentacles (the dorsal of Lacaze-Duthiers, the abaxial of Madrepora); 

 also, the small size of the tentacle on each side of this. Since the publication of Lacaze-Duthiers 

 results somewhat similar phases in the appearance of the prototentacles have been obtained in 

 other Actinians. In the Actinian, Lebrunia coralUgens, I have shown (1899) that a bilateral stage 

 with a large dorsal or sulcular tentacle is assumed even after a primary tetrameral radial phase. 

 Occasionally anemones are come upon in which the primary large tentacle is retained in the 

 adult, and in certain Sagartids occurring in Kingston Harbor the organ displays remarkable 

 motile powers. 



All the facts go to prove that the adult bilateral condition of the tentacles in Madrepora and 

 Porites is to he regarded as the retention of a larval stage occasionally passed through by 

 Actiniaria. 



METATENTACLES. 



No description is available as to the manner of appearance of the tentacles in any young coral 

 polyp beyond the two cycles of prototentacles. The few observations I have been able to make 

 indicate that the metatentacles appear practically simultaneously with the metacnemes, an exo- 

 ccelic and an entoccelic member together, as in Solenastrsea (fig. 83); or the entoccelic tentacle may 

 arise in advance of the exocoelic. as in the young polyp of Favia (tig. 109); Sid* rastrcea radians is 

 again exceptional in that its exocoelic metatentacles arise before the corresponding entocoelic 

 organs. 



Very definite accounts of the order of appearance of the tentacles in Actinia? are given by 

 Professor Lacaze-Duthiers (bs7i), and also by Dr. L. Faurot (1895). By these writers it has 

 been shown, in numerous instances, that the tentacles beyond the two first cycles arise in pairs, of 

 which one member is entocoelic and the other exocoelic. The entocoelic tentacle grows more 

 rapidly than the exocoelic, surpassing indeed the members of the outer (exocoelic) cycle of proto- 

 tentacles. The exocoelic metatentacle attains the same size as the exoccelic prototentacles, and 

 when the former are all developed the two series together are comprised in the third cycle, the 

 second cycle now being formed of the entoccelic metatentacles, which rank next in size to the 

 entoccelic prototentacles. 



The stages passed through will behest understood from the accompanying figures (fig. -1). 

 taken from Faurot's "Etudes." The process is that followed in Tealia felina. 



ORAL DISK. 



The oral disk is the more or less flattened distal termination of the polyp. It includes and is 

 bounded peripherally by the tentacular zone, and bears the oral aperture in the middle. In most 

 species the cycles of tentacles are closely arranged, and comprised within a narrow marginal 

 region, while the more central area of the disk, known as the peristome, is naked, and may be 

 depressed, flat, or elevated in a cone like manner. The tentacles on the disk of Siderastrsea and 

 Agaricia are comparatively widely apart, and the naked area is correspondingly diminished; the 

 tentacular zone in (>rhi<<ll<i ttcr<q><>rti also occupies a huge proportion of the disk (tig. .1, p. 423). 

 The discal walls are often delicate and partly transparent, anil permit of the septa being seen 

 through; like the column wall, the external surface may he smooth or verrucose. Usually 

 numerous radiating grooves occur, corresponding with the internal mesenteries: the grooves of 

 the complete mesenteries extend as far as the center of the disk, while those representing the 

 incomplete mesenteries stretch only part way. 



Lacaze-Duthiers; usually eight tentacles arise, practically simultaneously and equal. Knowing the great variability 

 often exhibited by Anthozoan larva?, according to the developmental stage at which they are extruded, it seems to 

 me not unreasonable to suppose that even the same species may presenl such wide variations as those given by 

 Lacaze-Duthiers and by Appellof. The agreement of Lacaze-Duthiers' figures of .1. equina with those representing 

 the tentacles of Madrcjtorn and Porites is certainly suggestive. 



