68 



PROTOZOA. 



ancient examples is perhaps somewhat dubious. As far as 

 we know at present, the Calcispongice commence in the Silurian 

 Rocks, attain their maximum in the Secondary Rocks, and are 

 diminished in numbers at the present day ; though Haeckel's 

 recent discoveries have rendered this last assertion more than 

 problematical. The Silicispongice. seem to have come into 

 existence during the Secondary period, attaining a great de- 

 velopment during the Cretaceous epoch, and being well repre- 

 sented at the present day. 



As regards Palaeozoic sponges, one of the earliest known 

 forms is the Archaocyathus (fig. 15) of Mr Billings, species of 

 which have been obtained from the Potsdam Sandstone and 

 Calciferous Sand-rock (Upper Cambrian?) of Canada. The 



general form in this genus 

 is that of a hollow cone or 

 hollow cylinder, enclosing a 

 large cup-shaped cavity, and 

 tapering towards one ex- 

 tremity, which was presum- 

 ably fixed to some foreign 

 body. Specimens appear to 

 have reached a very large 

 size, a length of two or three 

 feet and a diameter of three 

 or four inches being some- 

 times attained. The sponge 

 consists of an outer wall, 

 usually perforated with nu- 

 merous small irregular aper- 

 tures, and a thin inner wall 

 pierced with many open- 

 ings (fig. 15, a). The space 

 between the outer and inner 

 wall is subdivided into a 



number of vertical radiating partitions, thus very closely simu- 

 lating the structure of one of the septate corals. The genus, 

 however, is shown truly to belong to the Spongida by the 

 occurrence of numerous branching, cylindrical, or fusiform 

 siliceous spicula within the substance of the organism. In the 

 same geological horizon, and also in higher strata, occurs the 

 somewhat allied genus Calathium, in which the skeleton also 

 assumed a turbinate form. 



Amongst the Lower Silurian genera of Sponges may be 

 mentioned P-akzospongia, Acanthospongia, Eospongia, Palceo- 

 and Astylospongia (fig. 16). In the last, we have a 



Fig. 15. Restoration of the lower part of 

 A rchceocyathus M inganensis ; a the pores of 

 the inner wall of the cup. (After Billings.) 



