THE BRITISH FOSSIL BRACHIOPODA. 317 



such a unit as the length of human experience, is a very forcible instance to show that 

 the facts of zoology absolutely forbid the acceptance of any theory of necessary evolution 

 by continuous progress from homogeneity to heterogeneity/' 



Dr. J. F. Van Bemmelen says, in his excellent memoir on the ' Anatomy of the 

 Brachiopoda ' already referred to, that the results of his own investigations would lead him 

 " to confirm to some extent the opinion of the Hertwigs as to the Brachiopods being typical 

 enteroccelic animals." " The Brachiopoda," he adds, " are closely allied to the Chaeto- 

 gnathi. The great external differences between these animals are all attributed by him 

 to the influence of the shells, and so considered secondary changes ; while, on the 

 contrary, the features common to both are called by him chief or primary characters of 

 organisation. As such he mentions the similarity in development, already pointed out 

 by Butschli and the Hertwigs — the segmentation of the larvse into three portions — the 

 number, position, and origin of the generative organs, and their relations to the 

 nephridial efferent ducts — the perfectly similar structure of the nerve-collar with its two 

 centra — the ventral and dorsal longitudinal and the two pairs of transverse mesenteries. 

 To these facts he adds some remarks on the great similarity in histological structure 

 between the Brachiopods and Chsetognathes, such as the great simplicity of all the 

 epithelial layers, the subordinate significance of the connective tissue, the similar character 

 of the muscles, the plexiform distribution of peripheral nerves, and the occurrence of 

 horny setae in ectodermal follicles. The points of difference between Brachiopods and 

 Chgetognathes are explained as consequences of the development of the shell, which in 

 itself cannot be an argument against their affinity. This shell caused the development of 

 the peduncle, the arms, and the muscles, the removal of the anus to the right side or its 

 total disappearance, together with eyes, auditory organs (?), and jaws, and perhaps also 

 the unisexuality (to prevent self-fertilisation, while cross fecundation was secured by the 

 animals living in colonies). As support for this view the author points out the great 

 difference in plan and structure of peduncle, arms, and muscular system in different kinds 

 of Brachiopods, especially in Testicardines and Ecardines." 1 



In conclusion, I may perhaps be allowed to say that I have endeavoured briefly, and 

 as far as possible, in the different authors' own words, to fairly lay before the reader the 

 rather conflicting views that have been entertained with respect to this very important 

 and difficult subject. 



The most remarkable thing about all this theorising seems to be, not the relations of the 

 Worms and Brachiopods, which doubtless were derived from the same or from allied root- 

 forms, and which (as also in Mollusca, like JDentaliiim and the Pteropods) everybody may 

 see and accept, but the attempt to include the greater in the less. " Everybody," writes 

 Dall in a letter to me, " with a knowledge of systematic natural history, must know the 

 systematic value of the Brachiopoda, as a group, to be greater than that of the smaller 

 groups of Worms, in which it is proposed to include them. 



1 ' Annals and Mag. of Nat. Hist.,' vol. xi, 5th series, pp. 383, 384, 1883. 



