140 



PKOTOZOA. 



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 sometimes attained. The sponge consists of an outer 

 wall, usually perforated with numerous small irregular aper- 

 tures, and a thin inner wall pierced with many openings 

 (fig. 37, a). The space between the outer and inner wall 

 is subdivided by a number of vertical radiating partitions, 



thus simulating the structure 

 of one of the septate corals. 

 The genus, however, is cer- 

 tainly not a coral, and we 

 have the curious feature of 

 the occurrence of numerous 

 branching, cylindrical, or fusi- 

 form siliceous spicula within 

 the substance of the organism. 

 In the same geological hori- 

 zon, and also in higher strata, 

 occurs the somewhat allied 

 genus Calathium, in which 

 the skeleton also assumed a 

 turbinate form. According 

 to Dawson, the vertical la- 

 minse or septa in the upper 



portion of the cup of Archceocyathus are not only perforated 

 by numerous round apertures thus allowing contiguous 

 chambers to communicate freely with one another, but 

 they are themselves traversed by branching delicate canals 

 running in their substance ; and he regards the genus as 

 in some respects allied to Eozoon. Lastly, it is possible that, 

 in the Carboniferous genus Palceacis, often referred to the 

 Perforate Corals, we have in reality a type of calcareous 

 Sponges, with a vermiculate skeleton. 



3. THE SILICEOUS SPONGES (Silicispongice). In this 

 group are included those Sponges in which the skeleton is 

 made up of siliceous spicula or fibres. The skeleton may be 

 a discontinuous or continuous one, and in the living forms 

 the sarcode contains free siliceous spicules of microscopic 

 size and very variable form. These " flesh-spicules," though 



Fig. 37. Kestoration of the lower part of 

 Archceocyathus Minganensis. a, The pores of 

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



