418 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



muscle in the Madreporaria, having found the mesoglceal plaitings strongly developed in Sphen- 

 otrochus rvbescem. Gardiner (1900, p. 363) also describes a strong circular sphincter muscle in 

 ( 'a nopsammia. 



The sphincter, sometimes known as "Rotteken's muscle," is usually strongly developed in 

 Actiniaria, where it assumes very varied forms, and becomes of great importance for taxonomic 

 purposes. Actinian polyps in general are capable of retraction to a greater degree than are coral 

 polyps, but where no sphincter is present the disk and tentacles always remain exposed. There 

 is no doubt that the actual outline assumed by the mesoglceal plaitings supporting the fibrils, and 

 giving its character to the muscle, is largely dependent upon the amount of retraction and exten- 

 sion of the polyp, but still sufficient constancy remains to justify the importance attached to 

 the muscle for diagnostic purposes. 



A few observations upon the general expansion and retraction of coral polyps may lie here 

 given. * 



EXPANSION AND RETRACTION OF POLYTS. 



Only the more superficial tissues of coral polyps — column wall, disk, tentacles, and upper 

 part of the mesenteries — are capable of expansion and retraction, the change being brought 

 about mainly by the action of the musculature of these regions, with an accompanying entrance 

 or expulsion of water from the polypal cavity. The skeletotrophic tissues are destitute of muscle 

 fibers, and throughout remain adherent to the corallum. perhaps held in position by the peculiar 

 wedge-shaped or conical structures originating from the desmocytes (p. 4S"2); hence they take no 

 part in the varying aspects of the polyp. 



Polypal expansion proceeds slowly by the imbibition of sea water into the internal cavity, 

 and the consequent distension of the body wall. The musculature being relaxed, entrance of 

 the water is effected through the oral aperture, probably as a result of the activity of the 

 strongly developed stornodseal cilia. On retraction of the muscles, and subsequent diminution in 

 size of the polypal cavity, the water is largely expelled, also through the mouth. In a colony 

 where the ccelentera of all the polyps are in communication with one another, there seems no 

 reason why water should not be abstracted from one region to another, so that the polyps in 

 one part may be expanded and those in another retracted. The polyps of one area of a colony 

 are often in a different state of expansion from those of another. If an expanded colony be 

 suddenly lifted out of the water, flaccidity of the tissues almost immediately results, due to the 

 loss of water, and the latter can be actually observed flowing from the internal cavity. On irri- 

 tation of a single polyp in a fully distended colony the polyp readily retracts, and those around 

 more slowly, the water issuing through the mouth as a distinct stream. 



Polypal retraction is brought about by the united action of the musculature of the mesen- 

 teries, column wall, disk, and tentacles, the first mentioned being probably the most important. 

 The longitudinal retractor muscles are always more or less well developed on one face of each 

 mesentery, the mesoglcea being often folded to give increased area. By the contraction of these 

 muscles the distal region of the polyp is drawn downward; at the same time the contraction of 

 the circular endodermal musculature of the column wall aids in the shrinkage, and the same is to 

 be said of tin 1 circular musculature of the disk. 



From the comparative development and arrangement of the muscle fibrils throughout coral 

 polyps, it is manifest that retraction is entirely dependent upon muscular contraction, while 

 expansion is mainly due to the relaxation of the muscles, followed by the entrance of water. 



The external appearance of corals varies greatly, according as the polyps arc expanded or 

 retracted, and it is only from a full knowledge of both conditions that a clear understanding of 

 tin' relationships of the polyps to the corallum can be obtained. On complete retraction the 

 superficial tissues come to lie more or less closely upon the upper part of the corallum, always 

 separated, of course, from direct contact by the adhering skeletogenic tissues. In strongly 

 retracted examples of most species (lie costse and septa are seen through the polypal walls and 

 stand out prominently, and the tissues over them are much thinner than the portions of the wall 

 which occupy (he intervening depressions. Where the edges of the septa or costse are sharply 

 spinous, as in Isophyllia, the points appear as if perforating the tissues: but it may be doubted 



