392 



THE FORMS OF TISSUES 



[CH. 



meridian-like ciliated bands is a case in point (Fig. 175). Here, if we 

 imagine each Quadrant to be twice bisected by a curved anticline, 

 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 elliptical. 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 ciliated 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 hke manner I am inclined to suggest that the same principle 

 may help us to understand the apparently complex arrangement 



Fig. 176. Diagrammatic arrangement of partitions, represented by skeletal 

 rods, in larval Echinoderm (Ophiura). 



of the skeletal rods of a larval Echinoderm, and the very complex 

 conformation of the larva which is brought about by the presence 

 of these long, slender skeletal radii. 



In Fig. 176 I have divided a circle into its four quadrants, and 

 have bisected each quadrant by a circular arc (BC), passing from 

 radius to periphery, as in the foregoing cases of cell-division ; and 

 I have again bisected, in a similar way, the triangular halves of 

 each quadrant {DD). I have also inserted a small circle in the 

 middle of the figure, concentric with the large one. If now we 

 imagine those Unes in the figure which I have drawn black to be 

 replaced by solid rods we shall have at once the frame-work of an 

 Ophiurid (Pluteus) larva. Let us imagine all these arms to be 



