MATERIALS FOR A MONOGRAPH OF THE ASCONS. 491 



of course often occurs, and in which, after cutting sections, I 

 saw and figured the contracted pore-cells, though without 

 myself recognising their true nature. Had Topsent made and 

 studied similar preparations he would have been rather sur- 

 prised, I think, to have found his "cellules spheruleuses ^' 

 either completely absent or comparatively rare ; here and there 

 of course one finds a contracted pore in a specimen which, 

 as a whole, is expanded, and one generally also finds, without 

 difficulty, every transition between expansion and contraction. 

 It is possible that then Topsent would have been just as 

 '' moderately struck " by these cells as I was myself. 



I have gone into the question of these porocytes rather 

 fully ; but for this the importance of these porocytes, as we 

 shall see, from the point of view of spicule formation, as well 

 as the great confusion and diversity of opinion in the litera- 

 ture, must be my excuse. It is, indeed, a tangled web to 

 unwind when we try to introduce harmony into the existing 

 descriptions of these structures, nor can we expect that it should 

 be otherwise until spongiologists have more generally recog- 

 nised the excessive contractility of the species of Clathrina, 

 and the ease and rapidity with which they alter in appearance. 



Looking generally at the matter, we see that contracted 

 pore-cells, however interpreted, have been seen in a consider- 

 able number of species, more particularly in those which are 

 characterised by a very granular dermal layer, or, like 

 clathrus,by the granules being coloured. Thus Metschnikoff 

 saw them in clathrus and primordialis, but overlooked 

 them in blanca, where they are much less conspicuous. 

 Primordialis is a species which resembles coriacea in 

 being very granular; and in the latter Carter, Topsent, and 

 myself noticed these cells. Dendy observed them in cavata, 

 a species which his figures show to possess a very granular 

 dermal layer, and the only species in which Lendenfeld has 

 seen them beyond doubt is spinosa, a species which I have 

 myself had opportunity of studying at Banyals, and which 

 very closely resembles contorta, if it is not identical with it. 

 The only distinctive feature of contorta is the possession of 



