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Transactions of the Royal Canadian Institute 



length of a certain diameter in each plate for each winter period as shown 

 by the lines was then measured and the percentage that its length at 

 each winter period was of the present length was calculated. The average 

 value of the percentage for each winter period for the various plates was 

 taken and from this the length of the individual at each winter period 

 was obtained by calculation. We cannot claim any great accuracy for 

 this method, but it is sufficiently accuraite for our purpose. 



The character of the growth shown by these individuals varies 

 greatly. The annual increments in length may decrease steadily with 

 age, may decrease and then increase, may at first decrease, then increase 

 and finally decrease, or may at first increase and then decrease. On the 

 whole this species for this region shows a growth sustained with little 

 diminution for more than four years. 



Certain aspects of the growth are of considerable interest. The 

 superficial division of the disk into plates is seemingly a necessity arising 

 from the continuity and rigidity of the yellow layer. If the latter were 

 formed over the entire surface, increase in size would be impossible with- 

 out rupturing it and it might be doubted whether in view of the presence 

 of extensive cavaties in the animal connected with the exterior the pres- 

 sure due to growth could ever rupture the layer. The presence of definite 

 lines along which there is no deposition of yellow material and on either 

 side of which growth can take place, permits of the test enlarging with 

 the growth of the animal. These lines are made very definite by the 

 action of muscles. A characteristic feature of the life of an Ascidian 

 is its expansion, which is a preliminary to the commencement of feeding 

 by means of the water-current, and its contraction when disturbed. 

 These movements are effected through circular and longitudinal muscle 



