432 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



tig-ure of the larva, at the stage where twelve tentacles are present, sboulcl l)c compared with the 

 figui-es of the tentacles in the adult polyps of 2f(idrepora and Poriten^ on Pis. I and IV. It is seen 

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

 nence of one of the iixial tentacles (the dorsal of Lacaze-Duthiei's, the ahaxial of M(idr<porii)\ 

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

 results somewhat similar phases iti the ajjpearance of the prototentacles have been obtained in 

 other Actinians. In the Aetinian, Lchrunia coniUigenx. 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 Madrepm'a and 

 Por'di-K is ti> be regarded as the retention of a larval stage occasionally passed through Ijy 

 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 nietatentacles appear practically simultaneously with the metacnemes, an exo- 

 coelic and an entocielic m(>ml)er together, as in Solenafitrsea {Hg. 83); or the entoccelic tentacle maj' 

 arise in advance of the exoctelic, as in the young polyp of Favia (fig. 109); Sidrr((stnm. mdiaiin is 

 again exceptional in that its exoccelic nietatentacles arise before the corresjionding entoccelic 

 organs. 



Very definite accounts of the oi'der of appearance of the tentacles in Actinia? are given by 

 Professor Lacaze-Duthiers (1S72), and also by Dr. L. Faurot (1895). By these writers it has 

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

 whicli one member is entoc(plic and the other exocielic. The entoccelic tentacle grows more 

 rapidly than the exoccelic, surpassing indeed the menil)ers of the outer (exoc(elic) cycle of proto- 

 tentacles. The exocoelic metatentacle attains the same size as the exocoelic i)rototentacles, and 

 when the former are all developed the two series together are compi'ised in the third cycle, the 

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

 entoccelic prototentacles. 



The stages passed through will be best understood from the accompanying figures (fig. 2), 

 taken from Faurot's ''Etudes." The i)rocess is that followed in Tiidla fclina. 



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 CN'cles of tentacles ai'e closely arranged, and comprised within a narrow marginal 

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

 depres.sed, fiat, or elevated in a cone like manner. The tentacles on the disk of SUdemstriva and 

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

 tentacular zone in OrhtcelJa acropora also occupies a large proportion of the disk (fig. 1, p. 428). 

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

 through; like the column wall, the external surface ma\- be 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 repi'esenting the 

 incomplete mesenteries stretch only part way. 



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



often exhiliited by .\nthozoan larvpe, according to the developmental stage at which they are extruded, it seems to 

 me not unreasonal)le to supiiose that even .the same species may present such wide variations as those given liy 

 Lacaze-Duthiers and by Appelli'if. The agreement of Lacaze-Dutliiers' flgures of .1. eqttiiui with those representing 

 the tentacles of Madrepora and Porites is certainly suggestive. 



