Age-Determination, Growth and Symmetry 



33 



planation of this is to be sought in the fact that the animal to a con- 

 siderable extent lies upon its right side and ventral surface. This tends 

 to prevent growth of the disk towards the right and in front and to 

 encourage it towards the left and behind. 



The growth of the plates exhibits certain peculiarities. In a circular 

 plate symmetry of growth would be equal increment at all points along 

 the circumference; in a square plate equal increment on all four of the 

 sides; and in a rectangular plate equal increment on the two sides and 

 also on the two ends, but the increment at the ends greater than that 

 at the sides. The plates of Chelyosoma do not grow in this symmetrical 

 fashion, parallel sides of a plate not showing equal increment. The 

 asymmetrical growth of the individual plate is connected with its relation 



Figure 1 — Scheme showing genesis of plates in C. Macleayanum 



to the aperture to which it belongs. The effect may be stated thus, — the 

 amount of growth increases with the distance from the aperture. The 

 siphonal plates are shaped each like an isosceles triangle, of which the 

 apex forms the extremity of the apertural lobe. The superficial growth 

 of these plates is entirely at the bases of these triangles, that is at the 

 base of the siphon, the tips and sides of the plates not extending. The 

 extrasiphonal plates show greater ircrements on the sides next the margin 

 of the disk than on those next the siphons. There is, however, con- 

 siderable variation in the relative amounts of the yearly incVements on 

 the opposite sides of a plate, one side growing more, relatively to the 

 other side, in one year than in another. 



