20(5 HYATT, 



compose a deeply colored axis relieving and vivifying the 

 shadowy outlines of the tube and tentacular crest (W. 7, 

 fig. 5, K, K', K'0. 



All these delicately proportioned members are balanced 

 upon a fold of the endocyst, called the Invaginated Fold 

 (PI. 7, fig. 5, B), wrych is retained within the ecencecial 

 cell by the Retentor muscles (PI. 7, fig. 4, 5, N, N'). These 

 together with numerous other sets of small muscles will 

 be described hereafter. At present it is only necessary to 

 call attention to the Sphincter (PL 7, tig. 6, L), a broad, 

 contractile band surrounding the inva?inated fold, and the 

 large retractors (PL 7, fig. 4, fig. 8, M, M ; , M"), which are 

 in two sets, one on each side of the alimentary canal. 

 They arise apparently from two common bases, but each 

 large trunk subdivides above into many bundles, which 

 may be distinguished from each other according to the lo- 

 cation of their attachments arid divided into three branches. 



The fibres of the first branch, the Gastric Retractor, are 

 distributed to the stomach ; those of the second, the Lo- 

 phophoric Retractor, to the oesophagus and oral region ; 

 those of the third, the Brachial Retractor, to the bases of 

 the arms, and to the endocyst along the line of the Brachi- 

 al Collar. The crest is swayed by these muscles in every 

 direction ; or, when alarmed, the polypide may withdraw 

 by their aid into the larger coencecial tube below, very 

 much as the finger of a glove may be inverted within the 

 empty palm. This is so quickly done, at times, as to 

 baflle observation, and the fully expanded polypide, with 

 every tentacle stretched to its full length vanishes instant- 

 aneously within the coenoecium. Often, however, the in- 

 vagination is more slowly performed, and the motions can 

 then be easily followed. 



The polypidal endocyst is first turned inwards, folding 

 upon itself, and prolonging the permanently invaginated 

 fold below. The tentacles, arriving at the edge of the 

 coenoscial orifice, are pressed into a compact bundle by the 

 action of their own muscles, and, together with the lopho- 

 phore, are dragged into the cell by the continued invagina- 

 tion of the endocyst until they are wholly inclosed and at 

 rest within the sheath formed for them by the inverted 

 walls of the tube. The sphincter muscle then closes the 



