Branch A, Calcarea. 



Family I.— AsconiclEe. 

 " II.— Syconidse. 

 " III.— Leuconidse. 

 (Characterized respectively by the Canal 

 system indicated by the name). 



MICROSCOPICAL STUDIES. 39 



We may tabulate the larger divisions of the Sponges thus : — 



Phylum :— Porifera. 



Branch B, Non-Calcarea. 



Class I. — (Skeleton siliceous) Silici- 

 spongi^e. 



Order I.— Monaxonida. 



" II.— Tetractinellida. 



" III. —Hexactinellida. 

 Class II.— (Skeleton fibrous) Ceratosa, 

 Class III.— (Skeleton none)MYxosPONGi^} 



Putting aside in the present article any consideration of the 

 origin of the phylum as a whole, which is indeed bound up and 

 identical with that of the entirety of multicellular animals, it seems 

 most probable that the sponges arose independently by two main 

 lines from a primitive stock, rather than that the Non-Calcarea were 

 derived from the Calcarea or vice versa. Were the question limited 

 to the Calcarea, there would be little difficulty, for within the limits 

 of the division, forms are found ranging from the very simple Ascon 

 type, step by step up to the Sycon and thence to the complex Leucon 

 or Rhagon. The Non-Calcarea are on the other hand nearly all 

 of the Rhagon type, and were it not for the siliceous spicules in most, 

 might easily be derived from some form of the Calcarea. But the 

 nature of the spicules is a stumbling block. How derive a spicule 

 of flint (silica) from one of carbonate of lime ? Can we imagine the 

 spicule forming cells to suddenly alter their selective power and 

 to seize upon silica instead of lime ? It seems too much to ask. 



As to the Ceratosa, this difficulty is wanting and it is not 

 improbable that their origin is due to the renunciation of the 

 silica -secreting power in some ancestral siliceous sponge, while the 

 ancestors of the Myxospongise may have gone a step further and 

 have given up even the fibre forming power. But all is uncertainty 

 as yet. Even fossil records help us not at all, for the most highly 

 organized order, the Hexactinellida, are represented in our oldest 

 fossiliferous rocks, the Lower Cambrian, 



All the figures of entire sponges on Plate III are reproductions 

 of photographs taken from living specimens, and show very clearly 

 the outward form of four very prominent species of our Siliceous 

 sponges — Halichondria and Glathria (Figs. 1 & 2) armed with rod- 

 like spicules are typical forms of the Monaxonida : whilst the other 

 two, Tethya and Pachymatisma, are representatives of the great 

 group of the Tetractinellida. The third group, the Hexactinellida, 

 familiar to us in the lovely Venus' flower basket (Ewplectella), and 

 the strange Glass-rope sponge (Hyalonema) is not met with in 

 British waters. All belong to one or other variety of the Rhagon type 

 of canal system, and except in Tethya, the widely open mouths of the 



