THE CCELENTERAtES 



139 



body-cavity distinct from that of the stomach, but the two will be 

 seen to communicate below, since the stomach-wall does not extend 

 as far down as the base. It will be seen, too, that the body-wall is 

 made up of two distinct layers an outer one, that is continued 

 inward at the mouth to form the inner wall of the stomach, and an 

 inner one that lines the whole of the body-cavity. The latter 

 contains the muscular elements that enable the anemone to 

 contract its body. 



When the animal is expanded, the whole interior is filled with 

 sea water, as are also the tentacles, which are hollow tubes, really 

 extensions of the body- 

 cavity, and formed by 

 prolongations of the 

 same two layers that 

 constitute the body- 

 wall. As it contracts 

 this water is expelled, 

 partly through the 

 mouth, and partly 

 through small openings 

 that exist at the tips of 

 the tentacles. 



The outer layer of 

 the body-wall is pro- 

 vided with stinging 

 cells which serve not 

 only to protect the FIG. 90. SECTION OF AN ANEMONE 



anemone from its ene- * tentacles ; m, mouth ; *, stomach ; b e, body-cavity 



p, mesentery ; o, egg-producing organ 



mies, but also to aid it 



in the capture of its prey, for which latter purpose they are distributed 



in much greater abundance in the tentacles. 



The body-cavity is divided into a number of communicating 

 compartments by means of vertical partitions running from the 

 body-wall and converging towards the centre of the cavity. These 

 are called mesenteries, and are extensions of the inner layer of the 

 body-wall. Five or six of these are larger than the others, extend- 

 ing from disc to base, and are called primary mesenteries. Between 

 these are an equal number of smaller secondary mesenteries ; and, 

 sometimes, a third set of still smaller tertiary mesenteries. 



These internal partitions are best displayed in a transverse section 

 of the body, which shows the double tube formed by the walls of the 



