VIII] OF SUNDRY PARTITIONS 625 



we shall get what is apparently a close approximation to the actual 

 position of the cihated bands. The case however is comphcated 

 by the fact that the sectional plan of the organism is never quite 

 circular, but always more or less elhptical. One point, at least, is 

 clearly seen in the symmetry of the Ctenophores; and that is that 

 the radiating canals which pass outwards to correspond in position 

 with the cihated bands have no common centre, but diverge from 

 one another by repeated bifurcations, in a manner comparable to 

 the conjunctions of our cell- walls. 



In the early development of the shell (or "test") of a sea-urchin*, 

 each interambulacral area'consists of a lozenge of four plates, in the 

 famihar configuration assumed by four cells or bubbles, the polar 

 furrow lying in the direction of a " radius " 

 of the shell. A fifth plate, or "cell," 

 presently fits itself in between the third 

 and fourth, that is to say between the 

 terminal plate and one of the lateral ones, 

 and in doing so thrusts the former to 

 one side; a sixth intercalates itself be- ^'f' ^^^- / ''^"^^" .r"!, ^ 



lozenge : stages in the de- 

 tween the fourth and fifth, and so on velopment of the ambulacral 

 alternately. An ambulacrum consists of and interambulacral plates of 



two columns of calcareous plates, which ^ J^'^^^^i^- ^^' I- ^^'■ 

 fit into one another in the usual way by 



sutures set at angles of 120°. Each plate consists of three platelets; 

 a "primary" plate (a) is succeeded by a smaller and narrower 

 secondary plate (b) ; the squarish primary has one corner cut off by 

 a curved partition, to form a "demiplate", and the whole is called by 

 students of this group an " echinoid triad ". Though we do not know 

 precisely how the partitions arise, nor can we prove by measurement 

 their obedience to the laws of maxima and minima, yet their 

 general analogy to the principles we have explained is sufficiently 

 obvious. 



I am even inchned to think that the same principle helps us to 

 understand the arrangement of the skeletal rods of a larval 

 Echinoderm, and the complex conformation of the larva which is 

 brought about by the presence of these long, slender skeletal 



* Isabella Gordon, The development of the calcareous test of Echinus miliaris, 

 Phil. Tram.'iB), No. 214, p. 282, 1926; etc. 



