STRUCTURE OF THE POLYZOA. 183 



the fniiiculus or in the walls of the body. The body, 

 especially the lophophore, is retracted and pushed out 

 by muscles arranged in pairs on either side. As seen in 

 Fredericclla, a fresh -water form, the alimentary canal 

 "hangs from the lophophore, occupying the centre of 

 the polypide, and floating freely in the rapidly moving 

 blood" (Hyatt). The yellowish oesophagus, the stomach 

 barred with brown, and the brownish intestine are balanced 

 upon a fold of the intestine (the invaginated fold), which 

 is retained in the cell by the retentor muscles, and is sur- 

 rounded by a large sphincter muscle. There are two sets 

 of large retractor muscles, one on each side of the digestive 

 canal, and arising from two common bases ; each large trunk 

 subdivides into three branches, the retractor of the stomach, 

 of the lophophore, and of the anus. The crown of tenta- 

 cles is swayed by these muscles in every direction, or when 

 alarmed the polypide may withdraw by their aid into the 

 cell, as the finger of a glove may be inverted within the 

 empty palm. This may be done with great rapidity or 

 slowly. The process has thus been graphically described by 

 Hyatt: "The polypidal endocyst is first turned inwards, 

 folding upon itself, and prolonging the permanently invagi- 

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

 the coenoecial orifice, are pressed into a compact bundle by 

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

 lophophore, are dragged into the cell by the continued invag- 

 ination of the endocyst until they are wholly enclosed and 

 at rest within the sheath formed for them by the inverted 

 walls of the tube. The sphincter muscle then closes the 

 coenoecial orifice above, and the process of invagination is 

 completed. 



" The polypide in its exserted state is buoyed up and sus- 

 tained by the pressure of the fluids within. Consequently, 

 when invaginated, it displaces an equal bulk of these in the 

 closed coenoecium, and their reaction, aided by the contrac- 

 tion of the muscular endocyst, is sufficient to evaginate the 

 whole. 



" The evagination begins with the relaxation of the sphinc- 

 ter, which permits the ends of the tentacles to protrude. 



