STRUCTURE OF THE POLYZOA. 141 
the funiculus 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 
Fredericella, a fresh-water form, the alimentary canal 
‘thangs from the lophophore, occupying the centre of 
the polypide, and floating freely in the rapidly moving 
blood” (Hyatt). The yellowish cesophagus, 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 ccenecial 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 
concecial 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 ccoenecium, 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. 
