88 THE VOYAGE OF H.M.S. CHALLENGER. 



We may have another classification of the main groups of sponges if we consider as 

 the first principles of classification, not those important differences of the tubular and 

 vesicular canal-system (corresponding to the structures of the tubular and vesicular 

 glands), but the differences in the materials of the skeleton. The great value of these 

 skeletal differences has been acknowledged since Grant's time, and employed in various 

 ways by later authors up to this time, but it seems to me that no single author has 

 pointed out the important difference, phylogenetically, between a primary want of the 

 skeleton and a secondary one (by reduction) ; further, all authors of recent time, in my 

 opinion, have followed too far the monophyletic way (especially in judging of the Keratosa), 

 whilst in animals of such simple structure and low degree of organisation polyphyletic 

 hypotheses often approach nearer to the truth. 



Particular attention should be paid iu this respect to the Psammospongige, under 

 which name I comprise those remarkable so-called Keratosa in which no trace of spongin 

 is found, but in which the whole skeleton consists only of agglutinated xenophya, 

 crowded in the maltha, and is therefore a false or pseudo-skeleton. These Psanimospongise, 

 or pure arenaceous sponges, are represented in the Challenger collection by the 

 Ammoconidee (PI. VIII.) and Psamminidse (PI. VII.) described above, with six genera and 

 twelve species of peculiar interest. Most authors, following the presently accepted views, 

 would regard these Psammospongise as most reduced forms, derived from Silicosa, which 

 have lost the siliceous spicules as well as the spongin-skeleton. In my opinion, it is more 

 natural to regard these low forms as primitive ones, as Archispongise, which begin the 

 skeleton formation by taking up xenophya. 



Accepting this theory, we may even assume that the double formation of the 

 mineral skeleton of sponges, the calcareous and the siliceous, has a causal relation to the 

 double composition of the deep-sea ooze, from which the eldest Psammospongiae have taken 

 their skeleton materials, the calcareous Globigerina ooze and the siliceous Eadiolarian 

 ooze. The descendants of the oldest Archispongise (which certainly were skeletonless) 

 began to take up deep-sea ooze from the bottom, and to crowd this supporting and pro- 

 tecting material in their maltha. By and by the mesodermal tissue was adapted to 

 dissolve certain quantities of those two mineral bodies, and afterwards a certain portion 

 of the dissolved mineral matter contained in the maltha was secreted in the form of 

 spicules. This secretion may have been perfectly independent from the formation of 

 spongin-fibres in the maltha. In such a manner the oldest Calcispongise (Asconidse) may 

 have descended perhaps from Psammospongiae, which had taken up Globigerina ooze 

 {e.g., Ammolynthus haliphysema, PL VIII. figs. 2-4), and in a similar manner the 

 Silicispongise (perhaps polyphyletically in several independent branches) may have 

 arisen from the oldest Psammospongise, which had taken up Radiolarian ooze (e.g., 

 Ammolynthus prototypus, PI. VIII. fig. 1). 



Starting from this standpoint, we may accept as provisional the following classification 



