DR. T. DAVIDSON ON EECENT BRACIIIOPODA. 3 



The perplexing question of the affinities of the Brachiopoda has given rise to mucli 

 discussion, and great difference of opinion, especially \yith regard to their relationship 

 to the group of worms. Now, although I do not admit the Brachiopoda to be worms, 

 they may, as well as the Mollusca and some other groups of invertebrates, have originally 

 diverged from an ancestral vermiform stem, sucb as the remarkable worm-like mollusk 

 Neomenia Avould denote. In a recent paper on the development of Arylope or Cistella, 

 Mr. A. E. Shipley observes, and, I think, with justice, that the Brachiopoda and Polyzoa 

 are not so closely united as to form a natural phylum ; and he adds, " I should propose 

 to follow Gegenbaur in making a primary class of the Brachiopoda, and though in their 

 development and adult structure they are widely separated from botli Vermes and 

 Mollusca, of the two classes I would place them nearer to the former class than to the 

 latter " *. Prof. Huxley f says : — " All known Polyzoa are compound animals, that is to 

 say, the product of every ovum gives rise, by gemmation, to great assemblages of partially 

 independent organisms, or zooids. The Brachiopoda, on the contrary, are all simple, the 

 product of each ovum not giving rise to others by gemmation. All the Brachiopoda 

 possess a bivalve sliell — a shell composed of two, more or less horny, or calcified, pieces, 

 whicb are capable of a certain range of motion on one another, and are very commonly 

 articulated together by teeth and sockets." The shell, the pallial lobes, the intestine, the 

 nerves, and the atrial system, afford characters amply sufficient to define the class. 



In this view of Prof. Huxley I entirely concur. 



As many species of Brachiopoda live at considerable depths, it is not surprising that 

 so small a number should have been known to early conchologists, and that for many 

 years they should have been such groat rarities in conchological collections. The 

 numerous well-conducted dredging expeditions have, however, brought to light a large 

 number of forms that were not previously known, and we may constantly expect to add 

 to the number of species as dredging operations extend to regions not yet explored. 

 It has been ascertained beyond doubt that Brachiopoda are much localized, and that 

 where they occur they are generally abundant. It has also been found that the 

 range in depth of one and the same species is often very variable, that abyssal forms 

 have generally a very thin shell, and that species living at a great depth have a much 

 greater geographical range, and are not nearly so localized as those species that live 

 in shallow waters. 



The study of the species brought home by the ' Challenger ' Expedition, which I was 

 privileged to examine and describe, has revealed much valuable information with respect 

 to the Ijathymetrical and geographical distribution of many species. The greatest depth 

 at which a recent species of the class has been found alive was 2900 fathoms. A 

 number of forms inhabit and prefer rocky and stony parts of the bottom, or are attached 

 to corals, and are tlierefore more difficult to obtain. 



It is necessary briefly to refer to the difficult question of classification, upon which 

 many different opinions have been entertained. In company with a larger number of 



* •' On the Structure and Development of Arr/iojje." Mittheilungen aua der zool. Station zu Neapel, Band iv. 

 Heft 4, p. 516 (1883). 



t An Introduction to the Classification of Animals, p. 27 (18G9). 



