686 



ON CONCRETIONS, SPICULES, ETC. 



[CH. 



all probability, as Dreyer suggests, we have here had to do with a 



group of four vesicles, of which three were large and co-equal, while 



i\ fourth and very much smaller one lay 



above and between the other three. In 



certain cases where we have hkewise one 



large and three much smaller rays, the 



latter are recurved, as in Fig. 314, a-c. 



This type, save for the constancy of the 



number of rays and the limitation of the 



terminal ones to three, and save also for 



the more important difference that they 



occur only at one and not at both ends of 



the long axis, is similar to the type of 



V y 1 1 spicule illustrated in Fig. 312, which we 



\^ 1 ^f fl have explained as being probably de- 



^fflr J veloped within an oval cell, by whose 



■ ^ 11 walk its branches have been cabined and 



I I confined. But it is more probable that 



' we have here to do with a spicule de- 



Fig. 314. Spicules of tetracti- i • ^i • i ^ r r j.\. 



nellid sponges (after SoUas). veloped m the midst of a group of three 



a-e, anatriaenes; d-f, pro- co-equal and more or less elongated or 



tnaenes. cylindrical cells or vesicles, the long axial 



ray corresponding to their common edge or line of contact, and the 



three short rays having each lain in the surface furrow between two 



out of the three adjacent cells. 



Just as in the case of the little S-shaped spicules formed within 



the bounds of a single cell, so also in the case of the larger tetrac- 



tinellid types do we find the same configurations reproduced among 



the holothuroids as we have dealt with in the sponges. The holo- 



thurian spicules are a little less neatly formed, a little rougher, than 



the sponge-spicules, and certain forms occur among the former 



group which do not present themselves among the latter; but for 



the most part a community of type is obvious and striking (Fig. 315). 



The very peculiar spicules of the holothurian Synapta, where a 



tiny anchor is pivoted or hinged on a perforated plate, are a puzzle 



indeed; but we may at least solve part of the riddle. How the 



hinge is formed, I do not know; the anchor gets its shape, perhaps, 



in some such way as we have supposed the " amphidiscs "of -ff^^/^Zo- 



