,_ andFunctio7iofthe8j){cuIesof}ljalonema. 293 



coats like the rest of the body ; in fact they seem to be in- 

 creased in size by a layer of siliceous and animal matter being- 

 deposited on their whole sm-face, on the apices as well as the 

 rest of the body. 



On the conti-ary, the spicules of Hyalonema are subcylin- 

 drical, rather thicker above, and very slender at the base, 

 formed of numerous concentric laminje round a central line ; 

 but the outer layers do not reach the ends of the spicules. 

 The ends are truncated, and there is a succession of laminse, 

 each terminating shorter and shorter of the top of the spi- 

 cules ; so that the ends of the spicules are furnished with a 

 succession of rings, each formed by the termination of a layer 

 of siliceous and animal matter, as is well figured in the plates 

 of Professors Max Schultze, Brandt, and Bocage. 



I believe that this difference in the structure arises from the 

 very different functions which the spicules of the axis of Hya- 

 lonema have to perform in the coral from those for which the 

 spicules of the sponges are formed in the body of those beings. 

 In Hyalonema the coral is gradually being extended in height, 

 and the spicules are extended in Xtngth.^ pari passu ^ to support 

 the coral as it grows. To allow of this extension of length, the 

 ends of the spicules are not finished off as they are in the sponges, 

 where they are completed at once, and are only slightly 

 thickened and lengthened to a certain limit as the sponge 

 grows. The spicules of sponges merely form a support for a 

 more or less massive sponge, and are of a definite size ; while 

 the spicules of the rope-like axis are continually increasing in 

 length and thickness to support a continually growing mass 

 of animals, which require a larger and stronger axis to sup- 

 port them as the community assumes the adult form. The 

 spicules of the rope-like mass are of different thickness ; and 

 they increase in number as the axis increases in diameter, 

 those last formed being the most slender and consisting of the 

 fewest concentric layers. 



It is to be observed that the short rugose cruciform spicules 

 in the bark of the Hyalonema, which do not require to be 

 lengthened as the coral increases in size, are formed like the spi- 

 cules of sponges, and are complete in form when first deposited, 

 and have complete ends. This difference in the structure of 

 the spicules of the axis and of the bark, I think, goes far to 

 confirm the theory I have propounded, that the spicules of the 

 axis are of different structure, because they have a peculiar 

 function to perform. 



This peculiarity in the structure and function of the spicules 

 of the axis, I think, proves that they are no part of the sponge, 

 but a secretion of the hard flesh of the polype that surrounds 



