68 MANUAL OF ZOOLOGY. 



Similar root-fibres of fiint, traversing the mud in every direc- 

 tion, occur in the beautiful Venus' Flower-basket {Euplectella), 

 without any exception one of the most exquisite of all organic 

 structures known to us. 



Distribution of Sponges in Space. — Sponges are almost 

 exclusively marine, the Spongillce alone being inhabitants of 

 fresh water ; and they are of almost universal occurrence. The 

 sponges of commerce are mostly obtained from the Grecian 

 Archipelago and the Bahama Islands. Recently the existence 

 of numerous siliceous sponges at great depths in the ocean has 

 been demonstrated by Drs Carpenter and Wyville Thomson. 

 They are associated with numerous Foraminifera and with 

 Crinoidea, the whole assemblage bearing a singularly close re- 

 semblance to the fauna of the Cretaceous epoch. The com- 

 mon marine sponges are mostly found attached to some solid 

 object between tide-marks or in deep water. The vitreous or 

 siliceous sponges appear to be exclusively inhabitants of the 

 deeper parts of the ocean. One genus (Cliona) inhabits 

 branching cavities in shells, which the sponge excavates for 

 itself apparently by means of its siliceous spicula. Fossil 

 shells mined by a boring-sponge, allied to the recent Clionce, 

 are found from the Silurian rocks upwards. 



Distribution of Sponges in Timk. — Remains of sponges 

 are known to occur in formations belonging to the Palaeozoic, 

 Mesozoic, and Kainozoic epochs. The keratose or homy 

 sponges are obviously incapable of leaving any evidence of 

 their existence, otherwise than by the preservation of the 

 spicula with_ which the skeleton is 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 Petrospongiadm, 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 Spar- 

 sispongia (Devonian) and Ventriculites (Chalk). 



Of the Palseozoic sponges, Archceocyathus is found in the 

 Potsdam sandstone of North America (Upper Cambrian?); 

 Palceospongia and Acanthospon^a are familiar Lower Silurian 

 forms; 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 



