160 PHYLUM C@LENTERA. 
sometimes developed within the parent), some sea-anemones 
also multiply asexually by detaching portions from near the 
base, and fission occurs in a few forms. 
External appearance of a fixed Anemone. — The 
cylindrical body is fixed by a broad base; it bears whorls 
of hollow tentacles around the oral disc; the mouth is 
usually a longitudinal slit. The tentacles are contracted 
when the animal is irritated, and the whole body can be 
much reduced in size. Just below the margin of the oral 
disc there is a powerful sphincter muscle; this contracts, 
Fic, 78.—External appearance of Zealia crassicornis. 
and pulls together the body wall over the mouth and 
retracted tentacles. Water may pass out gently or 
otherwise by a pore at the tip of each tentacle, and long 
white threads, richly covered with stinging cells, can be 
ejected in many anemones through the walls of the body 
(Fig. 79). 
General structure.—- The Anthozoon polyp differs 
markedly from the Hydroid polyp—not only because an 
invagination from the oral disc inwards has formed a gullet 
tube, which hangs down into the general cavity, but also 
because a number of partitions or mesenteries extend from 
the body wall towards this gullet. Some of the partitions 
are “complete,” ze. they reach the gullet; others are “in- 
