424 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



protruded, while the rest are inturned. The process of introversion and subsequent extension 

 has been actually observed on the expanded polyp. The movements of the tentacular walls, 

 inwardly or outwardly, take place so slowly that they can be readily followed, the process some- 

 what resembling that seen when a glove finger is indrawn and afterwards pushed out. The 

 alternations may be continued for some time. Likewise on retraction of the polyps the tentacles 

 of Porites are. as a rule, introverted, instead of remaining- merely exposed or covered by the 

 column wall; and on preserved colonies very small apertures can be detected with a lens at 

 the place of introversion. In longitudinal sections through such polyps the apical knob of the 

 tentacle is deepest within the gastro-ccelomic cavity, and is directed outwardly, while with regard 

 to the walls themselves, the ectoderm is internal and the endoderm external, a reversal of the 

 ordinary condition. These conditions are clearly shown in tig. 40, representing a vertical section 

 through a polyp of Porites astrseoides. No overfolding of the column wall has taken place on 

 retraction, so that the tentacles communicate directly with the surface of the colony. Three 

 introverted tentacles (/) are present; the one to the left is divided radially, so that the section 

 includes its aperture of communication with the exterior, while the two to the right are tangential 

 sections, and therefore do not display the external opening. Again, in fig-. 30, representing a 

 transverse section through the stomodseal region of the same species, seven introverted tentacles 

 are seen in section, almost completely occupying the mesenterial chambers, and exhibiting 

 a reversal of the ordinal) relations of ectoderm and endoderm. in other polyps sectionized a 

 variable number of introverted tentacles has been met with. The apex of the introverted 

 tentacle may extend as far inwardly as below the inner termination of the stomodieum. so that 

 accompanying the introversion very little diminution in the length of the stem has taken place. 



Among the living expanded polyps of M(ulr<_p<>r<t also complete tentacular introversion is 

 often observed, in both apical and radial polyps. In the former six slight opacities around the 

 margin of the transparent disk remain to indicate the tentacular area; later, the tentacles may be 

 observed to protrude, either all together or successively. 



During the retracted condition of the fissiparous genera JFama, Mdnioina, Masandrina, and 

 Isqphyllia it is sometimes impossible to discover any tentacles externally. When sections are 

 made, however, the organs are found to be introverted, occupying both the entoccelic and 

 exoccelic mesenterial spaces. 



In addition to actual introversion, in which all parts of the tentacles are still determinable, 

 a condition is often presented in which the stem wholly disappears, becoming a part, as it were, 

 of the discal wall. 



Retracted tentacles of Si,/, rastrasa and Agaricia, for example, are usually represented by only 

 a slight tubercular elevation of the disk, which is the knob or swollen apex, while the stems have 

 wholly disappeared in the disk (PI. XXIII. tigs. 154, 155, and PI. XXIV, tig. 163). In micro- 

 scopic sections the former are displayed as mere ectodermal thickenings, charged with nemato- 

 cysts, and no differentiated can be found whereby the tentacular stem can be distinguished from 

 the discal wall. 



Among the fully expanded polyps of <_>rh',c,llii annularis the two cycles of short tentacles 

 often wholly disappear. Here, again, it appears as if the tentacular tissues were not introverted, 

 but rather have become involved in the greatly expanded margin of the disk; slightly raised. 

 triangular areas, representing the apical swellings, are all that can lie observed of the organs. 

 On full extension of the adult polyps of Mimicina areolata the tentacles likewise may be 

 wholly wanting, their walls having become part of the expanded disk. Thicker, more opaque 

 discal spots, which are the only evidence of their former presence, represent the nematocyst- 

 bearing capitulum. In the young polyp of Manicixtt displayed in section on PI. XIX, fig. 137, 

 the tentacle appears only as a thickened, nematocyst-bearing area of the polvpal wall. Occa- 

 sionally in I'uriiis astrseoides tentacular disappearance, as contrasted with tentacular introversion, 

 may be also observed. 



Of previous observers, Fowler (1888. p. 11) has described and figured the introversion of 

 the tentacles in Seriatopora subulata. Von Heider (1886, p. 158) has described in Astroides 

 calycularis the opposite condition, in which the intertentacular portions of the disk have been 



