Art. XL. — Development of the Brachiopoda. Part I. 

 Introduction ; by Charles E. Beecher, Ph.D. (With 

 Plate XYII.) 



The Brachiopoda have been so carefully studied, that any 

 new general conclusions regarding them must naturally be 

 based upon features not heretofore considered. In other 

 classes of animals, such important results have recently been 

 reached by the application of the law of morphogenesis as 

 defined by Hyatt, that the writer was led to study the Brachi- 

 opoda from this standpoint. The facts observed by this method 

 are mainly new to the class, and considerably affect the taxo- 

 nomic positions and affinities of the various families and genera. 



The value of the stages of growth and decline in work relat- 

 ing to phylogeny and classification is now generally admitted. 

 The memoirs of Hyatt, Jackson, and others, amply show that 

 the clearest and simplest understanding of a group may thus 

 be reached. The application of the principles of growth, 

 acceleration of development, and mechanical genesis, form the 

 main factors in the studies here made. The geologic sequence 

 of genera and species in this connection is also of the greatest 

 importance, for in this way the development of ancient species 

 may be studied, which in their adult condition represent 

 nealogic or nepionic stages of later forms. 



The prolific development of the Brachiopoda, both in point 

 of numbers and variety of genera and species, together with 

 their geological history, mark this group as one which should 

 furnish important data for the study of its genesis and of the 

 limits of a specialized variation in a single class. Moreover, as 

 its culmination was reached in paleozoic time, the group should 

 afford illustration of many principles of evolution. 



The main characters common to the class of Brachiopoda 

 are as follows : the bivalve shell ; the pedicled or fixed con- 

 dition; the animal composed of two pallial membranes inti- 

 mately related to the shell ; a visceral sac ; and two arms or 

 appendages near the mouth. The extreme range of vari- 

 ation does not eliminate any of these features, and, conse- 

 quently, no univalve or multivalve forms are found, nor any 

 strictly free swimming species, nor growths or modifications 

 adapting the organism to a pelagic life. Thus, the limits of 

 modification are narrowly restricted as compared with those of 

 several other classes, i. e. the Echinodermata and the Pelecy- 

 poda, but the thousands of known species of Brachiopoda show 

 what differentiation has taken place within these limits. 



Am. Jour. Sci. — Third Series, Yol. XLI, No. 244.— April, 1891. 

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