IV 



PHYLUM CCELENTERATA 189 



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

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

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

 jH'.rietal 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 

 downwards 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 re- 

 traction 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 

 the mesenteries and of their muscles is very definite and charac- 

 teristic (Fig. 137, B). At each end of the gullet, opposite the 

 siphonoglyphe, are two mesenteries (d. mes.), having their longi- 

 tudinal muscles turned away from one another : they are distin- 

 guished as the directive mesenteries, and, in the case of Tealia, 

 there are two couples of directive mesenteries, one at each end of 

 the long axis of the gullet. Of the remaining complete or 

 primary mesenteries there are four couples on each side (mes. 1), 

 differing from the directive couples in having the longitudinal 

 muscles turned towards one another. The secondary and tertiary 

 mesenteries (mes. 2, mes. 3) are also arranged in couples, and in all 

 of them the longitudinal muscles of each couple face one another. 



Symmetry. It will be noticed that Tealia, unlike the typical 

 hydrozoan and scyphozoan polypes, presents a distinct bilateral sym- 

 metry, underlying, as it were, its superficial radial symmetry. It 

 is divisible into equal and similar halves by two planes only, viz. a 



f u-a1 plane taken through the long diameter of the gullet, and a 

 tmmct: rse plane taken through its short diameter. 



