52 



THE CHAIN OF LIFE. 



evidence of horny or corneous sponges.^ The calcareous 

 sponges have been recognised as far back as the Silurian.^ 

 Thus from the close of the Palaeozoic all the types of sponges 

 seem to have existed side by side ; and in the Cretaceous period, 

 when such large areas of our continents were deeply submerged, 

 they attained a wonderful development, perhaps not equalled 

 in any other era of the earth's history. 



Fig. -^-i. — Oldhaniia antiqua 

 (Forbes). 



Fig. 34. — Dictyonetna sociale. Enlarged. 

 Lingula flags. — After Salter. 



Sponges may be regarded as the highest or most complex of 

 the Protozoa. We have no links wherewith to connect them 

 with the lower Protozoa of the Eozoic period ; and through 

 their long history, though very numerous in genera and species, 

 they show no closer relationship with the Foraminifera below, 

 and the Corals above, than do their successors in the modern 

 seas. They thus stand very much apart ; and modern studies 

 ^i their development and minute structures do not seem to 

 remove them from this isolation. Though we are treating 

 here of inhabitants of the sea, it may be proper to mention 



^ Protospongia (Salter). ^ Amphispongia, 



