230 STUDIES IN EVOLUTION 



ical genesis form the main factors in the studies here made. 

 The geologic sequence of genera and species in this connec- 

 tion is also of the greatest importance, for in this way the 

 development of ancient species may be studied, which in 

 their adult condition represent neanic 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 variation 

 does not eliminate any of these features, and, consequently, 

 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 modifica- 

 tion are narrowly restricted as compared with those of several 

 other classes; namely, the Echinodermata and the Pelecypoda, 

 but the thousands of known species of Brachiopoda show 

 what differentiation has taken place within these limits. 



The Protegulum. 



The first important observation to be noted is that all 

 brachiopods, so far as studied by the writer, have a common 

 form of embryonic shell, which niay be termed the protegu- 

 lum* The protegulum is semi-circular or semi-elliptical in 

 outline, with a straight or arcuate hinge -line, and no hinge- 

 area. A slight posterior gaping is produced by the pedicle 

 valve being usually more convex than the brachial. The 



* From irpii) early, and riyos a coveriug. 



