418 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



muscle in the Madreporaria, having found the niesogloeal plaitinji^.s strongly developed in Sj,/,, n- 

 otrochufi nihesceiis. (;;ir(liner (1!)0(). p. ;',(;:',) also deserilies a .stronii' cireiilur sphincter niiisclc in 

 CwnitpsaDrnua. 



The sphincter, soniftinics known as " Rotteken's nuiscle." is usually stronojy devcloix'd in 

 Aetiniaria, where it assuinc^s very varied t'ornis, and becomes of yreat importance for taxonomic 

 purposes. Actinian polyps in general are <'ai)al)le of i-etraction to a oreat(M- degree than arc coral 

 polyps, but where no sphincter is pi-esent the disk and lentacli>s always remain exposed. There 

 is no doubt that the actual outline assumed liy the mesogkcal plaitings sni)})orting the fibi-ils, and 

 giving its character to the nuiscle, is largely depi'ndent upon tln^ amount of retraction and exten- 

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

 the muscle for diagnostic i)iu'poses. 



A few observations upon the g(>neral expansion and retraction of coral polvps uiav be here 



given. 



KXPAXSIOX AND RETRACTION t)F POLYPS. 



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

 part of the mesenterie.s — are capable of expansion ancl retraction, the change being ))i'ought 

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

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

 fibers, and throughout remain adherent to the coralhuu, ixu-Jiaps held in position by the peculiar 

 wedge-shaped or conical structur(>s originating from the desiuocytes (p. 482); hence they take no 

 part in the varying aspects of the polyp. 



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

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

 the water is efl'ected through the oral aperture, probably as a result of the activity of the 

 strongly developed stomoda?al cilia. On retraction of the nuiscles. and subsequent diminution in 

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

 where the coelentera oi all the polyps are in conuuunication with one another, there seems no 

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

 one i)art may he exjjanded and those in another retracted. The polyps of one area of a colony 

 are often in a difierent state of expansion from thi:)se of another. If an expanded colony be 

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

 loss of water, and the latter can l)e actually observed flowing from the internal cavity. On ii'ri- 

 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 alnnit In' the united action of the nuisculature 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 mesoghea being often folded to give increased area. By the contraction of those 

 muscles the distal region of the l)olyp is drawn downward; at the same time the contraction of 

 the circular endodermal musculature of the column wall aids in the shiinkage. and the same is to 

 be said of the circular musculature of the disk. 



From the comparative development and arrangement of the nuiscle fit>rils throughout coral 

 polyps, it is manifest that retraction is entirely dej)endent upon muscular contraction, while 

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



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

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

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

 superficial tissues come to lie uiore or less clo.sely upon the uppei- ])art of the corallum, always 

 separated, of course, from direct contact b}' the adhering skeletogenic tissues. In strongly 

 retracted examples of most species the costfe and septa are seen through the polypal walls and 

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

 which occupy the intervening depressions. Where the edges of the septa or costa? are sharply 

 spinous, as in ludjiht/llin, the points appear ;!s if perforating the tissues; ))ut it may be doubted 



