ORGANIC DEPENDENCE AND DISEASE 29 



GENERAL SURVEY OF THE CAMBRIAN FAUNA OF 

 NORTH AMERICA 



The present registry of described species is now about 

 1200, and they range from algae to crustaceans and anne- 

 lids. This statement fairly represents the span of life in 

 this fauna. It is a reach from an expression of perfect 

 function with minimum of structural differentiation, as in 

 the sponges, to the specialized organic structure of the tril- 

 obite. 



Of the 1200 species, one-third (373) are brachiopods. 

 Brachiopods are animals which we believe to be derived 

 from a stock similar to, or identical with, that out of which 

 the worms have come ; and it is quite certain that the long- 

 lived ''inarticulate" brachiopods represented by Lingula, 

 retain pretty definite annelid resemblances. A vast num- 

 ber of Lingulas occur in this fauna and their form of at- 

 tachment, if comparable with the living Lingula anatina, 

 was like that of many contemporary worms — a burial in the 

 mud, rather than a fixation to the sea bottom. The great 

 array of Cambrian brachiopods presents at maturity a min- 

 imum of fixation by means of the pedicle, which was an 

 organ not homologous with the byssus by which the mussel 

 shells are attached but an adapted organ obviously of a 

 different original function. Throughout the later Paleo- 

 zoic story of these brachiopods, attachment by the pedicle 

 was easily surrendered, and solid fixation by the substance 

 of the shells easily assumed. The fact is to be emphasized 

 that the brachiopods are a distinct order of creatures with 

 no affiliations with the Mollusca and none except in sem- 

 blance with the Molluscoida. 



Of the Mollusca which swarmed in the Postcambrian seas, 

 but few had then been developed or at least have been regis- 

 tered: less than 10 per cent of the whole fauna, and but 3 

 per cent of these are of the dependent type of the oyster 



