78 THE ELEMENTAEY NERVOUS SYSTEM 



digestive cavity, but is held in place by membranes, the 

 mesenteries, which extend in pairs from the inner face 

 of the cylindrical wall of the actinian's body, the col- 

 umn wall so-called, across the digestive space to the wall 

 of the oesophagus. By means of these mesenteries the 

 oesophagus thus comes to be held in an axial position in 



the actinian's body, where 

 it serves as the one means 

 of inlet and outlet for the 

 digestive cavity. Although 

 it is a relatively simple 

 tube it is usually provided 

 with a pair of longitudinal 

 grooves, the siphono- 

 glyphs (Fig. 18), by which 

 water is continually pass- 

 ing into the interior of the 

 sea-anemone to escape in 

 an outward current 

 through the rest of the 

 oesophagus. 



The mesenteries, as can best be seen in a transverse 

 section of a sea-anemone (Fig. 18), are thin sheets of 

 tissue which, as already mentioned, occur in pairs. The 

 members of each pair are separated from each other only 

 by a very narrow space, the entocele, which is really an 

 extension of the digestive cavity. Each pair is separated 

 from the pair on either side of it by a wider space, the 

 exocele. The mesenteries that hold the oesophagus in 

 place extend, as already stated, from that structure to the 

 column wall and are known as complete mesenteries. 

 Those that are attached to that portion of the oesophagus 

 that forms a siphonoglyph have a peculiar arrangement 



Fio. 18. Transverse section of the sea- 

 anemone Metridium showing the oesophagus 

 e with its two siphonoglyphs s and its support- 

 ing mesenteries, the directives d and the com- 

 plete non-directive c. Two series of incom- 

 plete mesenteries i are shown. 



