118 G. H. PARKER 



for Sagartia, an actinian as it creeps leaves behind it a trail of 

 slime much as a snail does. This slime is produced by the many 

 unicellular glands in the ectoderm of the pedal disc and undoubt- 

 edly serves to increase the adhesion between the disc and the sub- 

 stratum. When the resting place of one of the more sessile 

 actinians, like Metridium, is examined, it is usually found heavily 

 coated with slime, which is often partly solidified into a mem- 

 brane of a more or less brownish color. In assuming this rela- 

 tively fixed condition the pedal disc of Metridium gradually 

 spreads out over a comparatively large area and by the secretion 

 of slime the actinian may anchor itself very firmly to the sub- 

 stratum. In this operation the region of most effective attach- 

 ment is the edge of the disc rather than its center. Neverthe- 

 less the disc does not appear to act as a sucker, a fact which is 

 probably correlated with the absence of a parieto-basilar muscle 

 in this species. 3 Thus the pedal disc of Metridium, like the 

 foot of the gastropod, may serve as a holdfast as well as an organ 

 of locomotion (Parker, '11). In this respect, however, the disc 

 must not be looked upon as a fleshy organ that simply adheres 

 to the substratum in a purely mechanical fashion. It has often 

 happened that the large actinians, Metridium and especially 

 Condylactis, when in full locomotion, would meet with difficulty 

 in loosening the active part of their discs from the substratum. 

 Most of the disc would easily become free, but particular points 

 would often continue to adhere to the substratum until after 

 much straining they would suddenly break away. The impres- 

 sion made at the time was that these points of adhesion repre- 

 sented minute well localised organs of attachment, but whether 

 such really exist in addition to the general surface of the disc I 

 can not say. 



IV. STRUCTURE OF THE PEDAL DISC 



The parts directly concerned with the creeping of actinians 

 are the muscles in the pedal disc and in the immediately adja- 

 cent region as well as the fluid contained therein. In Metridium, 



3 In other species of actinians in which there are parieto-basilar muscles the 

 pedal disc may act as a sucker as surmised by Pieron ('06), Herouard ('11) and 

 others. 



