720 
CLASS XII. 
Owen On the Anatomy of the Brachiopoda; b. pp. 145—164, 
Pl. 22, 23 (both these memoirs are translated and reprinted in the 
Ann. des Sc. Nat., sec. Série, m1. 1835, pp. 26—30 ; 52—77). 
R. Owen Lettre & M. Minne Epwarps sur appareil de la Cireu- 
lation chez les Mollusques de la classe des Brachiopodes, Ann. des Sc. 
Nat., 3ime Série, ut. 1845, Zool. pp. 315—320, Pl. 4. 
W. Kine Remarks on certain Genera belonging to the Class Pallio- 
branchiata, Annals of Natur. History, xvi. 1846, pp. 26—42; 
83—94. 
OweENn On the Anatomy of Terebratula, Introduction to the British 
Fossil Brachiopoda by DAvipson. Printed for the Paleeontographical 
Soc. 1854, pp. 4—22. 
The chief characteristic of this division consists in the respira- 
tory organs. In Ligula, on each lobe of the mantle on the inside, 
two main trunks are seen that run longitudinally and return the 
blood to the heart, which is double, as we have seen above, at the 
part where the two are most remote, and converge obliquely to the 
free margin of the lobe of the mantle in the form of an inverted V. 
Lateral and parallel branches, that make a right angle with the 
trunk, form an elegant pectinated design on the lamine of the 
mantle. It is these vascular divisions which compose the respiratory 
organs ; the gills are, therefore, attached to the mantle, or rather the 
mantle itself is the seat of respiration. From this disposition that 
which presents itself in Orbicula and Terebratula differs more in 
appearance than in reality. The respiratory function is not limited 
here, as in Ligula, to a part of the mantle, but is distributed over 
the whole of it, whilst two vascular stems on one lobe and four on 
the other are formed from numerous branches, and bring back the 
arterial blood to the double heart (OwEn Z'ransact. Zool. Soc. 1.. 
pp. 147, 148, 154). The two hearts do not receive the blood 
immediately, however, in these molluscs, from the stems of the 
branchial veins, but these last pour it into a swus, from which it is 
taken up by the free and wide opening of the two auricles of the 
heart (Ann. des Sc. Nat. dieme Serie, m1. pp. 316—319). 
A second character of this order is afforded by the two so-named 
arms, which lie rolled up with their convex side outwards near the 
mouth, and are margined with cirri or filaments. In TJerebratula, 
where these filaments are long and fine, these two arms thus acquire 
the external appearance of gills, and have accordingly been so named 
by earlier writers, ew. gr. Pattas Miscell. Zool. 1766, p. 182, 
