MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 435 



MOUTH AND STOMOD/EUM. 



Tin- actual form of the mouth of corals depends much ii|i<>ti the condition of expansion or 

 retraction of the polyp. In the retracted or partly ret '-acted state the aperture, asa rule, is nar- 

 row and slit-like while the outline assumed on expansion may be nearly circular. Under certain 

 conditions the mouth is closed all the way. with the exec] it ion of a small opening at each extremity. 

 In practically all cases a longer and a shorter axis are determinable, thus giving a bilateral char- 

 acter to polyps which otherwise would he outwardly radial in symmetry. In genera like Mamoina 

 anil Mseandrina, with a meandering disk, bearing numerous small oral apertures, the longer 

 axis of the latter is usually along the length of the disk, and the shorter axis is transverse. In 

 branching colonies the longer oral axis is approximately in the axial-abaxial plane, while in 

 the many polyps of compact flattened colonies it may lie either radial or irregular in direction 

 with regard to the middle of the colony. 



The usual condition of living polyps is one in which the mouth is partly open, the white, 

 smooth, depending walls of the stomodseum easily distinguishable through it. Hounded lips 

 sometimes serve as a gradual transition from the disk to the stomodseum, but in deeply pigmented 

 species the boundary between the disk and stomodseum is usually very sharply defined. When 

 polyps are retracted, the mouth is generally in the same plain' as tin' flattened disk, hut on 

 expansion it becomes more or less elevated along with the central part of the peristome (fig. 46). 



The stomodseum is usually oval in transverse section, hut may he circular. Its vertical 

 extent, as a rule, is comparatively short, more so than is usually the case in Actinian polyps. 

 In some specie-, the lower stomodseal edge can he easily discerned when the mouth is widely open, 

 the organ suggesting a mere inturned flap of the disk. Sometimes the walls of the stomodseum 

 are smooth, hut in perhaps the majority of species they are thrown into deep vertical ridges and 

 furrows, extending the whole length of the organ, and a little less marked in the fully expanded 

 than in the retracted state. Generally the ridges are more noticeable on the living polyps than 

 after preservation, and those of opposite sides alternate. 



When the polypal tissues are partly transparent, the stomodseal ridges are seen to correspond 

 in number and position with the attachment of the mesenteries to the inner or ccelomic surface 

 of the stomodseum (Fl. XIX. tig. 131). To a limited extent, therefore, they serve to indicate the 

 number of complete mesenteries. The ridges are found to he very variable in number in forms 

 such as Mssandrina, Manicina, and tsopkt/Uia, which happen to he species in which they are 

 besl developed. In Maeandrina, for example, only three or four ridges will he present on each 

 side of the stomodseum where the oral aperture is small, while in others there may he seven or 

 eight. 



On transverse section the stomodseal ridges are seen to lie formed by thickenings of the 

 mesogloea, and less so of the ectodem, hut the endoderm takes no part (Fl. XXII. tig. 147). In 

 species in which the ridges are best developed the ectoderm of the elevations exhibits a slight 

 histological distinction from that of the furrows; large nematocysts and gland cells occur among 

 the supporting cells of the former, while they are practically absent from the intervening areas, 

 which on their part are more strongly ciliated. At the inner termination of the stomodseum the 

 ridges appear as if continued down the free edge of the complete mesenteries as the mesenterial 

 filaments, and the histology of the two agrees very closely. 



With the exception of the ridges and furrows, occurring only in certain species, the stomo- 

 dseal walls are structurally uniform all round: in other words, true gonidial grooves or sipho- 

 noglyphs are absent from Madreporarian polyps. As met with in the Actiniaria, at the opposite 

 ends of the stomodseum, the gonidial grooves are readily distinguished in the living condition by 

 the greater thickness and firmness of the walls, and by their smooth free surface; histologically 

 the ciliation is stronger than elsewhere, and usually nematocysts and glandular cells are 

 more sparingly distributed. The grooves in anemones are invariably associated with a pair of 

 directive mesenteries. 



