70 Miss Agnes Crane — Evolution of the Brachiopoda. 



the heart and vascular system, and by the restriction of the pedicle 

 opening to one valve of the shell. We can trace in the primitive 

 members of the order Neotremata one manner in which this change 

 originated. In the elongated forms of the non-perforated shells of 

 the Atremata the long peduncle passes out freely between two valves. 

 In some of the earlier forms a small notch is developed in that valve 

 which becomes eventually the pedicle valve. In the primitive 

 Neotremata this notch deepens, widens as in Discinolepis, and in 

 Orbiculoidea and Discina it has become almost centrally situated, 

 for the shell grew up later and enclosed the shorter byssus or 

 mooring organ (PI. V.' Figs. 12; IQ a, b, c; 20). 



Hence a long peduncle is usually associated with elongated or 

 oval shells, a solitary life, and free axial movement. A short byssus 

 yielded less space to move in, and gi'owth became concentrical. 

 Thus the "disc shells" are co-ordinated with the short mooring 

 organ. Both existing members of the " newly perforated " order 

 (Neotremata) adhere in masses to each other or to rock-surfaces, 

 and the consequent crowded condition of their environment affected 

 the shape and growth-direction of the individuals. In Crania, which 

 cling in clusters by their lower valves to rock-surfaces, the margins 

 of the central shells cannot expand and the shell grows more in an 

 upward direction ; hence we get the conical helmet shells resembling 

 a French cap of liberty. The divergence between the outer and 

 inner shells of such a mass of individuals produces what some would 

 consider as almost specific differences. 



Such are some of the sad effects of overcrowding among the 

 Brachiopoda. These serve as instances of the mechanical origin 

 of genera and species. 



The history of the class is almost coeval with the century. Before 

 that time the Brachiopoda were generally regarded as curiosities; 

 or, if their molluscau character was recognized, they were usually 

 grouped in that class of Lamellibranchiate bivalves which it is 

 once again the fashion to call Pelecypoda. Cuvier was the first to 

 determine correctly the natural affinities of the Brachiopoda (23). 

 He placed them in a class by themselves at the base of his sub- 

 kingdom, Mollusca, and gave them their not over-exact name of 

 Brachiopoda, which Dumeril subsequently adopted. Milne-Edwards 

 classed them later with the Polyzoa and Tunicata in a separate 

 sub-kingdom to which he gave the name of Molluscoida, or the 

 mollusc-like animals. 



The actual affinities of the Brachiopoda have been the subject of 

 much debate and discussion. There is now a general tendency to 

 revert to the classification of Cuvier. The early worm-like larval 

 stages considered by Steenstrup, Morse (64, 55), and Kowaleveski 

 as proof of the vermian affinities of the Brachiopoda are regarded 

 as " recapitulatory phases " indicative of the parallel descent from 

 one common source of the widely-divergent animals belonging to 

 the sub-kingdoms Annulosa and Mollusca. 



1 These figures refer to PI. V., which will accompany the conclusion of the article 

 when the Bibliography will be given. — A. C. 



