INVERTEBRATA. 19 



voyagers to the extremities of the southern continents, and at 

 the greatest elevation hitherto climbed in the Andes or Hima- 

 layas. If some classes — e.g., Tunicata, Acaleplm — seem not to 

 be represented in stratified deposits, they are such as, from 

 the soluble or perishable tissues composing the entire frame, 

 at least under one of their metagnetic phases, could not be 

 expected to be fossilized under any conceivable circumstances. 

 Evidence, however, of compound Hydrozoa — i. e., of the 

 polypes which Ellis called "Corallines" — and especially of 

 the genus Campanularia, would shew that the acalephal type 

 and grade of organization had been manifested at the period 

 of the formation of the strata containing such fossil Polypi.* 

 With the above seeming exceptions, every class of inverte- 

 brate animal is represented by fossil remains. 



They consist of corals and shells, of the crusts of star- 

 fishes and sea-urchins, of the coverings of crabs and insects, 

 of the tracks and shelly habitations of worms, and of impres- 

 sions of surfaces and casts of cavities of soft invertebrates, 

 retained by the matrix after the animals had perished. 



The condition in which such fossils occur depends on the 

 nature of the matrix and other accidental circumstances ; for 

 while some are scarcely altered in composition, or even in 

 Colour, others are silicified or infiltrated with carbonate of 

 lime, and every part of the original may have been dissolved 

 away and replaced by another mineral substance, atom after 

 atom, in the rock which contained it. These evidences of 

 former life may come into view by fracture of the rock or 

 by exposure to the weather ; farther insight may be gained by 

 the action of acid ; and some require the chisel of the mason 

 or the mill of the lapidary for the proper exhibition of their 

 structure. 



Multitudes of recent species are fossilized in the newer 

 tertiaries whose history can be made out perfectly from living 



* Owen, "Lectures on Invertebvata," 1855, p. 159. 



