720 CLASS XII. 



OWEN On the Anatomy of the Brachiopoda ; ib. pp. 145 1G4, 

 PL 22, 23 (both these memoirs are translated and reprinted in the 

 Ann. des Sc. Nat., sec. Serie, in. 1835, pp. 2630 ; 5277). 



R OWEN Lettre a M. MILNE EDWARDS sur Tappareil de la Circu- 

 lation chez les Mollusques de la classe des Brachiopodes, Ann. des Sc. 

 Nat., 3ieme Serie, in. 1845, Zool. pp. 315320, PL 4. 



W. KING Remarks on certain Genera belonging to the Class Pallio- 

 branchiata, Annals of Natur. History, xvni. 1846, pp.26 42; 

 8394. 



OWEN On the Anatomy of Terebratula, Introduction to the British 

 Fossil Brachiopoda by DAVIDSON. Printed for the Palaeontographical 

 Soc. 1854, pp. 422. 



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 Y. 

 Lateral and parallel branches, that make a right angle with the 

 trunk, form an elegant pectinated design on the laminae 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 Transact. Zool. Soc. i. 

 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 sinus, from which it is 

 taken up by the free and wide opening of the two auricles of the 

 heart (Ann. des Sc. Nat. 3ieme Serie, in. pp. 316319). 



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 Terebratula, 

 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, ex. gr. PALLAS Miscell. Zool. 1766, p. 182, 



