140 DR A. MILNES MARSHALL ON THE 



The section also shows four zooids cut, like the polyps, in different planes 

 and at different levels. 



Plane of Symmetry. — Each polyp of a Pennatulid colony can be divided 

 longitudinally into two perfectly similar halves by one plane only, which is 

 spoken of as the plane of symmetry. This plane passes between the two long 

 mesenterial filaments, bisecting the septal chamber bounded by the two septa 

 which bear these filaments ; it also bisects the septal chamber immedi- 

 ately opposite to this one, and passes along the long axis of the stomodasum, 

 which in transverse section (fig. 28, s) is oval, not circular in shape. In 

 Kophobelemnon this plane of symmetry of each polyp has a very definite 

 relation to the rachis. The plane is a vertical one, and is perpendicular to the 

 surface of the rachis to which the polyp is attached, so that if prolonged it 

 would pass through the centre of the calcareous axis or stem. These relations 

 will become more obvious from an inspection of fig. 28. In the case of all three 

 polyps shown in this figure, the planes of symmetry, being vertical when the 

 specimen is placed upright in its natural position, will be at right angles to the 

 plane of the paper. In the case of the dorsal polyp the plane of symmetry 

 must pass through the centre of the polyp cavity, and must also (by definition) 

 pass midway between the two long mesenterial filaments p ; it is obvious from 

 this figure that this plane, if prolonged, will pass through the centre of the 

 calcareous stem c. 



So also in the case of the right hand polyp of the figure. In order to 

 divide the retractor muscles rm symmetrically, it is clear that the plane of 

 symmetry must bisect the septal chamber next to the stem c, and also the 

 chamber immediately opposite to this one ; such a plane will pass along the 

 longer axis of the stomodaeum s, and will, if prolonged, pass through the 

 centre of the stem c. 



So with all the other polyps, the plane of symmetry will always be a 

 vertical one, will be at right angles to the surface of the rachis at the point of 

 insertion of the polyp, and will, if prolonged, pass through the centre of the 

 calcareous stem. 



It is further evident from fig. 28 that the two long mesenterial filaments are 

 on the side of the polyp next to the stem, so that the surface of the polyp 

 which, when the polyp becomes free from the rachis (cf. fig. 23), is continuous 

 with its upper surface, may conveniently be called the axial surface; while the 

 opposite surface, which is furthest from the stem, and which is continuous with 

 the lower surface of the polyp when this becomes free from the rachis, may be 

 called the abaxial surface. 



I have already used the terms axial and abaxial when describing the 

 surfaces of the Pennatula zooids," and have done so in exactly the same sense 



* Supra, p. 125. 



