726 Mr, Edward A, Minchh [May 6, 



form and arrangement of the spicules have a definite relation to its 

 structure. In the simplest cases only triradiates are present, and 

 then they are arranged in a single layer, all placed with one ray 

 pointing downwards, away from the opening at the top. The rays 

 of different spicules overlap and cross one another, and so produce a 

 sort of lattice-work, with meshes rather like a honeycomb. In the 

 meshes are placed the pores, and at first the arrangement is such 

 that there are the same number of pores and spicules, the result 

 being that each spicule has a pore in each of the interspaces between 

 the arms. As the sponge grows, however, new pores and new spicules 

 are constantly being formed, so that the simple arrangement is upset 

 to some extent, though the same general pattern can be made out. 

 When an extra fourth ray is added on to the triradiate system, it is 

 always placed so as to project into the cavity, and if the extra ray 

 is curved, it always points up towards the large opening at the top. 

 If simple needle-shaped spicules are present thev are always placed 

 on the outside, with the straight portion of the shaft embedded in the 

 wall, and the curved portion sticking out into the water. 



The relation of the spicules to the structure of the sponge shows 

 that they have a definite function to perform and an important part 

 to J)lay in the economy of the organism that has produced them. 

 Their function is partly one of support, partly one of protection. 

 Given a vase-like organism, with a thin porous wall, what are the 

 architectural requirements of a supporting and protecting framework 

 for it, supposing that for the material of the framework rods of cal- 

 cite are to be employed ? The simplest solution of the problem would 

 be to place the rods in the body wall, so that one or more come to lie 

 between each of the pores. Such an arrangement would, however, 

 be far from jierfect, since on the one hand a skeleton of loose uncon- 

 nected rods is not very strong, and on the other hand it does not 

 afford any protection. Hence the next stej) in the evolution of the 

 framework is, on the one hand, to bend some of the rods so that 

 they point outwards, and so cover the outside with a forest of sharp 

 spikes ; and, on the other hand, to join up some of the loose rods in 

 the wall and unite them into composite systems. Now of all the 

 systems that could be devised by joining rods together, none could 

 be more suited to the type required than the triradiate figure pro- 

 duced by joining three rods only. In the first place each triradiate 

 corresponds perfectly to the natural interspaces between the pores, 

 which if disposed so as to best economise sj)ace, take on an arrange- 

 ment m alternating rows, so that each pore is surrounded by six others 

 at equal distances, forming a hexagon. In short, the arrangement of 

 the pores repeats the familiar problem of the angles of the cells of 

 the honeycomb, and the triradiate spicules correspond exactly to the 

 interspaces. Secondly, it must be remembered that the sponge has 

 to live in waves and currents, and its framework requires a certain 

 amount of flexibility as well as strength. This condition also is best 



