SEA-ANEMONE. 121 



they be used to indicate bilateral symmetry? Make a 

 drawing of the animal showing the column, oral disc, etc. 

 Cut off a few tentacles and see if they be hollow or solid. 



INTERNAL STRUCTURE. 



Cut the animal with a sharp knife into two portions, 

 the incision being made parallel to the oral disc and pass- 

 ing through the body about a quarter of an inch from the 

 oral 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 intraradial 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 



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

 ment of the septa; these exceptional forms should be compared 

 with the more normal specimens. 



