iv PHYLUM CCELENTERATA 187 



within, and between them an intermediate layer or mesogloea, 

 which is extremely thick and tough. The gullet (gul.}, which, like 

 that of the scyphula, is an in-turned portion of the body-wall, is 

 lined with ectoderm, and its outer surface i.e. that facing the 

 inter-mesenteric chambers is endodermal. The mesenteries (mes.) 

 consist of a supporting plate of mesogloea, covered on both sides by 

 endoderm. The tentacles (t) are hollow out-pushings of the disc, 

 and contain the same layers. 



Muscular System. Sea-anemones perform various charac- 

 teristic movements : the column may be extended or retracted, the 

 tentacles extended to a considerable length, or drawn back and 

 completely hidden by the upper end of the column being folded 

 over them like the mouth of a bag ; the gullet, and even the 

 mesenteries, may be partially everted through the mouth ; and 

 lastly, the whole animal is able, very slowly, to change its position 

 by creeping movements of its base. 



These movements are performed by means of a very well- 

 developed set of muscles. A mesentery examined from the surface 

 is seen to be traversed by definite fibrous bands, the two most 

 obvious of which are the longitudinal or retractor muscle (Fig. 

 138, l.m.), running as a narrow band from base to disc, and the 

 parietal muscle (p.m.), passing obliquely across the lower and outer 

 angle of the mesentery. Both these muscles are very thick, and 

 cause a projection or bulging on one side of the mesentery, 

 specially obvious in a transverse section (B, l.m.) : a third set of 

 fibres, forming the transverse muscle (t.m.), crosses the longitudinal 

 set at right angles, but is not specially prominent. The longi- 

 tudinal muscles shorten the mesentery, and draw the disc 

 do\vnwards or towards the base, thus retracting the tentacles ; the 

 parietal muscles approximate the column to the base, and the 

 transverse fibres produce a narrowing of the mesentery and thus, 

 opposing the action of the longitudinal muscles, act as extensors of 

 the whole body. The withdrawal of disc and tentacles, during 

 complete retraction, has been compared to the closure of a bag by 

 tightening the string, and is performed in much the same way, the 

 string being represented by a very strong band of fibres, the 

 circular or sphincter muscle (s.m.), which encircles the body at the 

 junction of the column and disc. 



The foregoing muscles can all be seen by the naked eye, or 

 under a low magnifying power. They are supplemented by fibres, 

 only to be made out by microscopic examination, occurring both in 

 the body-wall and in the tentacles. The latter organs, for instance, 

 are able to perform independent movements of extension and 

 retraction by means of delicate transverse and longitudinal fibres. 



It was mentioned above that the thickness of the longitudinal 

 and parietal muscles produces a bulging on one surface of the 

 mesenteries. A transverse section shows that the arrangement of 



