146 



CCELENTERA. 



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, 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 anem- 

 ones through the walls 

 of the body (Fig. 64). 



General structure. — 

 The Anthozoon polype 

 differs markedly from 

 the Hydroid polype — 

 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 mesen- 

 teries extend from the 

 body wall towards this 

 gullet. Some of the 

 partitions are " com- 

 plete," i.e. they reach 

 the gullet; others are 

 " incomplete," i.e. do 

 not extend so far inwards. The complete mesenteries 

 are attached to the oral disc above, to the side of the 

 gullet, and to the base, and all the mesenteries are in- 

 growths of the body wall. The cavity of the anemone 

 is thus divided into a number (some multiple of six) of 

 radial chambers. These are in communication at the base, 



Fig. 64. — Vertical section of a sea- 

 anemone. — After Andres. 



i., Tentacles ; o., mouth ; a?s., oesophagus ; c, c'. t 

 apertures through a mesentery; a., a'., acontia ; 

 g., genital organs on mesentery ; m.f., mesen- 

 teric filaments; ?;?./., longitudinal muscles; 

 s., primary septum or mesentery ; s'., second- 

 ary septum; /'., tertiary septum; v., basal 

 disc. 



