48 THE CHAIN OF LIFE. 



already in the Silurian period the sea was tenanted with all the 

 forms of invertebrate life it yet presents, and these in a teeming 

 abundance not surpassed in any succeeding age. Let us now, 

 in accordance with our plan, select some of these ancient 

 inhabitants of the waters and trace their subsequent history. 



Remains of sea-weeds are undoubtedly present in the Cam- 

 brian rocks. One of the lowest beds in Sweden has been 

 named from their abundance the Fucoidal Sandstone ; and 

 wherever fossiliferous Cambrian rocks occur, some traces, more 

 or less obscure, of these plants may be found. Nearly all that 

 we can say of them, however, is, that, in so far as their remains 

 give any information, they are very like the plants of the same 

 group that now abound in our seas. In the fucoidal sandstone 

 of Sweden certain striated or ribbed bodies have been found, 

 which have even been regarded as land plants ; ^ but they seem 

 rather to be trails or marks left by sea-weeds dragged by cur- 

 rents over a muddy bottom. The plants of the sea thus 

 precede those of the land, and they begin on the same level 

 as to structure that they have since maintained. 



The Foraminifera of the Palaeozoic we have noticed in the 

 last chapter ; but we now find a new type of Protozoan — that 

 of the Sponge. Sponges as they exist at present may be de- 

 fined to be composite animals, made up of a great number of 

 one-celled or gelatinous zoids, provided with vibrating threads 

 or cilia, and so arranged that currents of water are driven 

 through passages or canals in the mass, by the action of the 

 cilia, bringing food and aerated water for respiration. To 

 support these soft sarcodic sponge-masses, they secrete fibres 

 of horny matter and needles (spiculae) of flint or of limestone, 

 forming complicated fibrous and spicular skeletons, often of 

 great beauty. They abound in all seas, and some species are 

 found in fresh waters. 



With the exception of a very few species destitute of skele- 

 ton, and which we cannot expect to find in a fossil state, the 

 ^ Eophyton Linnaanum (Torrell). 



