REPORT OF THE DIRECTOR I9IO 



225 



of r.post.B and r.ant.R. A new factor would now enter to deter- 

 mine the position of the fourth b.v. Nos. 2 and 3 would be its 

 nearest neighbors. It would find water richer in oxygen and 

 freer from excretory matters in the direction of no. i. Its own 

 effort to function to better advantage might alone insure for it a 

 position on the suture next no. i, but natural selection would soon 

 fix any variation in this direction however caused. To follow the 

 series in this manner down to no. 12 and find a probable cause 

 for each position, is a simple matter. The evidence seems to 

 favor the idea that the b.vs. of a triangle are organically related 

 to each other and are but external branches of one internal tube 

 or chamber. There is no evidence, however, to warrant our 

 associating the group with the circumoral ring of the water vas- 

 cular system though a connection was not at all unlikely. 



It is of course possible that nos. i, 2 and 3 were independent 

 b.vs. and that, as the sutures were extended, each politely awaited 

 its turn to send a branch through the point of least resistance. 

 Such a condition could be made to speak eloquently against the 

 idea of struggle for existence between parts of an organism, but 

 there seems to be no evidence in its favor. An examination of 

 the sutural faces of the plates might decide the question and free 

 plates of this species may yet be found and examined. If the 

 development was as at first suggested, the sutural canals should 

 show an inclination toward a point under the plate corner and 

 that would be the position of the larger subthecal canal or sac 



Fig. II From a photomicrograph of a plate of a species of Palaeocystites, seen from .the edge 

 The edge of a steel mm rule shows just below. 



into which the b.vs. would periodically discharge their contents. 

 If the development and grouping was of the other type, the 

 8 



