BRITISH SPONGE OF THE GENUS RAPHI0DE8MA. 101 



ment, and as all sponges commence by a membranous expansion, it 

 is interesting to take particular note of this feature. The hair-like 

 spicules have a fascicular arrangement though laid in flat bands, the 

 spicules parallel to each other. A few of the skeleton spicules, 

 without any defined order, cross these irregularly, as well as a few 

 of the finer kinds which belong specially to the membranes, and in- 

 terspersed, as previously stated, are scattered small anchorate spicules 

 (Fig. 3). Upon this membrane the sponge developes its fasci- 

 cules, proper to the skeleton in that regular order described as be- 

 longing to this and to the sponges alluded to, as congeners. 



We are a long way as yet from any cognisance of the laws which 

 separate one species from another, but there is no reason why they 

 should not be similar to those operative in the vegetable kingdom. 

 A slight shift of a plant to fresh earth is sometimes sufficient to pro- 

 duce changes which would move many observers to talk of a 

 separate species, so that it is not only possible, but probable, the 

 development of a sponge under either restricted conditions of growth, 

 or of a more exuberant fertility, may equally change its nature and 

 produce a variation. I have already shown how great is the variation 

 with our freshwater sponge {Spongilla fluviatilis), and there can 

 be no reason why it may not operate extensively amongst the marine 

 sponges ; I cannot help thinking that, if more accurate knowledge 

 was obtained, it would reduce the number of species. It is, 

 to my mind, far more interesting and more instructive to combine 

 together groups under the name of variation, than add to our 

 already overweighted terminology by a persistent separation. I 

 shall, therefore, now call upon you to follow me in an examination of 

 the whole of the genus Raphiodesmay to show how closely they 

 are allied, and how much of their divergencies may arise from slight 

 differences of conditions under which they have developed. 



Three, out of four of the species tabulated by Dr. Bowerbank, 

 are marked by possessing those beautiful rosettes formed of clusters 

 of anchorate spicules. In the species under consideration, one of 

 these also occurs, thus making no doubt of its close alliance (Fig. 

 6). But in mode of growth, as spreading over thinly, it is nearest to 

 that oiR.Jloreum, separated from it, however, as from all others, in the 

 want of the bihamate spicule. This latter form unites R. jioreum 

 directly with R. sordida, indirectly with R. lingua, wherein these are 

 found so extremely minute as to require high powers for examination. 

 R. Jioreum wants the tricurvate spicule oi R. sordida, or it is difficult 

 to see how, in their structural details, you could separate tlie one 



