Sixth Section. 



Brachiopoda. 



General Review of the Group. 



§ 238. 



Tlie Brachiopoda, whicli by most authors were formerly placed with 

 the MoUusca, with which they have little in common save a shell, 

 which is quite different from the one found in the Mollusca, form a 

 small and well-defined division, which is derived from the phylum of 

 the Vermes. Among the Vermes the Chgetopoda, which are highly 

 differentiated forms, appear to have several points of affinity with 

 them; but nothing more, for there are such striking peculiarities 

 even in the most important systems of organs, that it would be going 

 too far to base any definite phylogenetic assertion on these relations. 

 In any case the whole organism of the Brachiopoda, as compared 

 with the Chastopoda and Annelides, is completely metamorphosed, 

 and genetic relations can only be made out in a few rudiments. 



This eminently isolated position of the Brachiopoda at the 

 present time corresponds to the slight variations seen in the extant 

 forms, as well as to the fact that we have here to do with a group 

 of animals which was richly developed in earlier periods. Some 

 genera are found as early as the Silurian epoch. But as the palae- 

 ontological evidence is not sufiicient to justify us in associating them 

 with the Vermes, it is better to treat them separately than to unite 

 them with those forms.* 



We distinguish two orders only. 



1) Ecardines. 



Lingula, Orbicula, Crania. 



2) Testicardines. 



Terebratula, Argiope, Waldheimia, Theciclium. 

 * I rccoguised their affinity to tlie Vermes in my " Grundzuge," IT. Anfl. 



