PROTOZOA: SPONGIDA. 75 



their existence, otherwise than by the preservation of the 

 spicula with which the skeleton is sometimes furnished ; and 

 such are occasionally found, though they are of rare occurrence. 

 The calcareous sponges are found from the Silurian rocks 

 upwards, attaining their maximum in the seas of the Secondary 

 epoch, the Chalk being especially characterised by their 

 presence. The most important group of fossil sponges is that 

 known as the Petrospongiadce, characterised by the possession 

 of a stony reticulate framework or skeleton, and by the absence 

 of spicula. The most important genera of this group are 

 Sparsispongia (Devonian) and Ventriculites (Chalk). 



Of the Palaeozoic sponges, Archceocyathns is found in the 

 Potsdam sandstone of North America (Upper Cambrian ?) ; 

 Palaospongia and Acanthospongia are familiar Lower Silurian 

 forms j and Amphispongia and Favospongia occur along with 

 other forms in the Ludlow rocks. In the Devonian rocks 

 sponges occur pretty frequently, Sparsispongia being the com- 

 monest genus. (The Devonian Steganodictyum is really the 

 cephalic buckler of a pteraspidean fish.) The most important 

 Mesozoic genera of sponges are Ventriculites and Siphonia ; 

 and the order appears, upon the whole, to attain .its maximum 

 in the Cretaceous epoch. There seems no reason to doubt 

 but that many of the chalk-flints owe their origin to sponges ; 

 and in some sections of flint are found minute " spherical bodies 

 covered with radiating and multicuspid spines," which have 

 been termed Spiniferites or Xanthidia, and are probably the 

 " gemmules " of sponges. (By some, however, these bodies are 

 regarded as being the " sporangia " of Desmidice, an order of 

 the Protophyta^) Many Cretaceous and Tertiary shells are 

 found to be mined by a species of boring-sponge, which is 

 nearly allied to the recent Cliona. 



AFFINITIES OF SPONGES. As already pointed out, the 

 sponges are allied both to the Amoeba and to the Foraminifera. 

 Indeed the individual "sarcoids" or sponge-particles can scarcely 

 be distinguished, when detached, from Amcebce. The sponges 

 show likewise a decided relationship to the Radiolaria ; and by 

 Professor James Clark they are believed to be nearly allied to 

 the " flagellate " Infusoria. This observer, in fact, states his 

 " conviction that the true ciliated Spongia are not Rhizopoda in 

 any sense whatever, nor even closely related to them, but are 

 genuine compound flagellate Protozoa." To prove this view, 

 however, it should be shown that each sponge-particle possesses 

 at any rate a distinct mouth, with or without a rudimentary 

 alimentary canal. More recently Dr Ernst Haeckel and others 

 have endeavoured to show that the sponges are most nearly 



