MATERIALS FOR A MONOGRAPH OF THE ASCONS. 561 



of the sponge as a whole, receives further support from the 

 fact mentioned ahove, namely, that in the case of the triradiate 

 systems first secreted after the fixation of the larva, extreme 

 irregularity prevails. The sponge has not at this stage assumed 

 its cylindrical form, but is a compact mass of cells; the spicules 

 are formed in the superficial layer just as in the adult, and 

 were their regular form due to the arrangement of the secreting 

 cells, there is no reason why the rays should not at this early 

 stage meet at the same constant angles as in the adult. It is 

 not, however, until the young sponge has assumed its charac- 

 teristic tubular form that regularity becomes the rule, and not 

 the exception, amongst its spicules. I can only interpret these 

 facts by supposing that the forms of the spicules are influenced 

 by the requirements of the sponge as a whole, and not by the 

 arrangement of its cells, so that the spicules do not attain 

 their characteristic form until the sponge itself has the structure 

 to which that form is an adaptation. And this conclusion 

 seems to me still further supported by the above-mentioned 

 differences between Clathrina and Leucosolenia. In the 

 former the reticulate form of the sponge colony is associated 

 with the possession of equiangular triradiates, which in the 

 latter become bilateral in correlation with the upright form of 

 the sponge as a whole. 



If the triradiate systems furnish a strong argument, as I 

 believe they do, for the influence of adaptation on the spicules, 

 the course of events in the growth of a quadriradiate system 

 greatly strengthens this impression. We have seen that after a 

 triradiate system has been laid down in the usual way, the 

 gastral ray is tacked on to it from quite a different source, 

 namely, by a porocyte. We have seen, further, that the poro- 

 cytes come from the dermal epithelium, and we know that in 

 Leucosolenia each cell of this epithelium can secrete a 

 monaxon spicule. Hence there is no difficulty in supposing 

 that the porocyte in secreting a monaxon spicular element is 

 simply exercising a function which was primitively possessed 

 by every cell of the dermal layer. We can invoke the aid of 

 such well-known principles as heredity, atavism, and even 



