DISSECTION OF A SEA-ANEMONE. 297 



ing through the body about half an inch from the mouth 

 end. In the upper portion (i.e., that nearest the oral 

 disc) will be found an oesophagus extending inwards from 

 the mouth. Can you trace the siphonoglyphes into this 

 tube ? Extending inwards from the outer wall to the 

 oesophagus are six * pairs of partitions, the primary mesen- 

 teries or septa. The result of this is that the space inside 

 of the body is divided into a series of chambers. The 

 chambers between the septa of a pair are called the intra- 

 radial, those between the pairs of septa the interradial 

 chambers. The interradial chambers will be found to be 

 partially subdivided by other pairs of septa (secondary, 

 tertiary, etc.) which extend outwards from the body- wall, 

 but which do not reach the oesophagus. 



Examine the primary septa and find on each a muscle 

 extending in the direction from oral disc to base. Are 

 these muscles on the inter- or iritra- radial sides of the septa ? 

 Examine all the primary septa before deciding this ques- 

 tion. Then sketch the cut surface, inserting body-wall, 

 oesophagus, and primary septa ; and on each of the septa 

 put the muscles on the proper surface. If this be done, it 

 will be found that there is but one plane which will divide 

 the animal into exactly symmetrical halves. The septa 

 through which this plane passes are the directives. Do 

 they correspond in position to the siphonoglyphes ? Study 

 a few of the incomplete septa. Have these muscles like 

 the others ? At the oral ends of the septa look for open- 

 ings through these partitions. 



Split the other part of the animal lengthwise, and pin out 

 under water. Notice that the oesophagus does not reach 



* Occasionally variations will be found in the number and arrange- 

 ment of the septa ; these exceptional forms should be compared with 

 the more normal specimens. 



