Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/230

BRACHIOPODA

THE Brachiopoda constitute an important and well- defined class of Invertebrates, but the exact position the group should occupy in that division of the animal kingdom is still a matter upon which anatomists have not entirely agreed. For many years the species composing the class were referred to the genus Anomia of the Lamelli- branchiata, but, as was judiciously observed by Edward Forbes, " a close examination shows that there is no rela tionship between them, but only a resemblance through formal analogy." Milne-Edwards separated the Mollusca into two great divisions, Mollusca and Molluscoida, and in the last he placed the Brachiopoda, Polyzoa, and Tunicata, an arrangement that has been followed by many naturalists. Although the greater number of zoologists have admitted the close connection existing between the Polyzoa and Brachiopoda, considerable doubt has been expressed with respect to the affinities and position of the latter in relation to the Tunicata ; moreover, a strenuous effort has been made within the last few years by Steen- strup, Morse, Kowalevsky, A. Agassiz, and others, to de monstrate that the affinities of the Brachiopoda and Polyzoa are with the Worms, and that they should form classes of Annulosa, and be placed close to the Annelids.

According to Agassiz, the transition between such types as Pedicellina to Membranipora and other incrusting Polyzoa is readily explained from the embryology of Thecidium, and, in fact, all incrusting Polyzoa are only communities of Brachiopods, the valves of which are con tinuous and soldered together, the flat valve forming a united floor, while the convex valve does not cover the ventral one, but leaves an opening more or less ornamented for the extension of the lophore. Both Gratiolet and Hancock have expressed the opinion that the Tunicata are in no way related to the Brachiopoda, and that we cannot place these last and the Polyzoa along with the Tunicata in the same division. Gratiolet and some others have considered the Brachiopoda to be allied to the Crustacea, while even the asteridian affinities of the class have been hinted at by King.

No doubt can be entertained, after perusing the admirable memoirs by Morse atid Kowalevsky on the embryology of Terebratula, Terebratulina, Aryiope, and Thecidium, that the genera composing the class and Amphetrite possess many important features in common, but almost any Inver tebrate groups might be annelidilized by overrating certain points in their affinities. Mr Dall thinks that the general conclusion with reference to the affinities of the Brachiopoda will be something like this. There is much reason for supposing that all the Molluscs and Molluscoids came from the stock out of which the Worms have developed. Indeed, as Huxley has said, they are only isomerous Worms with many special modifications. It is natural, therefore, that the oldest and lowest forms should retain many of the characteristics of the oldest and most simple Worms, espe cially those which have been modified by a tubular habit. But, on the whole, the modifications are so important that we may continue to consider (if in the specializing tendency of present study we can retain any general divisions of Invertebrates) that the Molluscoids and Molluscs do form two groups somewhat aside from others, and somewhat more nearly related to each other than to the divisions external to them. Therefore, although it may turn out that the Brachiopoda constitute a class close to the Annelids, it cannot be denied that they possess many molluscan characters that cannot bo overlooked, and are, under any circumstances, entitled by their importance and numerous distinctive features to constitute a well-defined and separate class.

The name Brachiopod (/3/ja^twv, an arm, TTCWS, TroSJs, X:un. a foot) was proposed for the class by Cuvier in 1805, and by Dumeril in 1809, and has since been very extensively adopted. Blainvillc in 1824 proposed as a substitute for the Cuvierian name that of Palliobran- chiata (pallium, a mantle; branchiae, gills), on account of the respiratory system being combined with. the mantle on which the vascular ramifications arc distributed. Prof. King has always adopted the latter name, and perhaps rightly objects to Cuvier s on the ground that it is a misnomer, for the two variously curved and cirrated brachial or labial appendages, improperly designated us arms or feet, were subsequently found not to subserve the function of locomotive organs.

FIGS. 1-8. Clistenterata.

FIGS. 9-11. Tretenterata.

FIG. 1. Waldlteiniia eraneum. A, ventral, B, dorsal valve

FIG. 2. Rhynchonella psittucea. FIGS. 3 and 4. Thecidium. FIG. 5. Spirifer. Dorsal valve, showing calcareous spiral cops. FIG. 6. Orthis calliyramma. Fio. 7. Lepta-na transversalis. A, ventral, B, dorsal valve. FIG. 8. Producing horridus. FIG. 9. Lingula pyramidata (after Morse) FIG. 10. Discina lamellosa. FIG. 11. Crania anomala. Interior of dorsal valve, showing muscular impressions and labial appendages

Before describing the various parts of the animal and TWO divi-

its shell, it may be as well to mention that it had been sious. many times suggested by Owen, Bronn, Huxley, Gill, and others, that the class could be advantageously divided into two primary groups. Thus, for the first division, including Llngula, Discina, &c., the names Lyopomata (Owen, 1858), Pleuropygia (Bronn, 18G2), Inarticulata (Huxley, 15 GO), Lyopomata (Gill, 1871), have been made use of; while for the second division, comprising Tertbratida, lllnjn- ckonella, &c., the names Athropomata (Owen, 1858),

Apygia (Bronn, 18G2), Articulata (Huxley, 18GU), Arthro-