DR. T. DAVIDSON ON EECEXT BRACIIIOPODA. 49 



somewhat convoluted and branched ; they are of a full yellow colour, and are thrust 

 into the trunks and main branches of the great pallial sinuses" (p. 817). 



" The heart is a simple, unilocular, pyriform vesicle, suspended from the dorsal aspect, 

 and projects freely into the perivisceral chamber " {loc. cit. p. 831). 



Prof. Huxley, in his ' Introduction to the Classification of Animals,' 18G9, says that 

 " The precise characters of the true vascular system of the Brachiopoda probably 

 require still further elaboration than they have yet received ; and the same may be 

 said, notwithstanding the valuable contributions of P. Midler and of Lacaze-Duthiers, 

 of their development ; but the shell, the pallial lol^es, the intestine, and the nervous and 

 the atrial systems, afford characters amply sufficient to define the class " (p. 30). He also 

 says (p. 29), " In all Brachiopoda which have been carefully dissected, a singular system of 

 cavities and canals situated in the interior of the body, but in free communication \\i\\\ the 

 surrounding medium, has been discovered. This, which I shall term the ' atrial ' system 

 (from its close coi'respondcnce with the system of cavities, which has received the same 

 name in the Ascidians), has been Avrongly regarded as a part of the true vascular system, 

 and the organs by which it is placed in communication with the exterior have been 

 described as ' hearts.' There are sometimes two and sometimes four of these ' pseudo- 

 hearts,' situated in that part of the body-wall which helps to bound the pallial chamber. 

 Each pseudo-heart is divided into a narrow, elongated, external portion (the so-called 

 ' ventricle '), Avhich communicates, as Mr. Hancock has proved, by a small apical aperture 

 with the pallial cavity ; and a broad, funnel-shaped, inner division (the so-called ' auricle '), 

 communicating, on the one hand, liy a constricted neck with the so-called ' ventricle,' 

 and on the other, by a wide, patent mouth, with a chamber which occupies most of the 

 cavity of the body proper, and sends more or less branched diverticula into the pallial 

 lobes. These have been described as parts of the blood vascular system; and the 

 arterial trunks, Avhich have no existence, have been imagined to connect the apices 

 of the ventricles with vascular networks of a similarly mythical character, supposed 

 to open into the branched diverticula. In fact, as Mr. Hancock has so Avell shown in 

 his splendid and exhaustive memoir published in the ' Philosophical Transactions ' for 

 1857, the true vascular system is completely distinct from this remarkable series of 

 ' atrial' chambers and canals, the function of which would appear to be to convey away 

 excretory matters and the products of the reproductive organs, which are developed in 

 various parts of the walls of the atrial system." 



In an exquisitely beautiful enlarged illustration in his ' Anatomy of the Terebratiila,' 

 Owen represents the bracliial aponeurosis and spii*al arms of JJ^aldheimia Jlavcscens, 

 showing the central part of the nervous system, with the brachial and the beginning 

 of the pallial nerves. 



27. WALBHEniiA VEXOSA, Solaudcr, sp. (Plate YIII. figs. 1-5 ; Plate IX. fig. 1.) 



Anontia venosa, Solander, G. Dixon, A Voyage round the World, Appendix no. 1, p. 355 and fig., 1789. 

 Terehrutula glohosa, Valenciennes, apiid Lamarck, An. sans Vert. vol. vi. j). 2ifi, 1819, with 

 reference for figui-e to Encyclop. Method, p. 239, figs. 5 a, b, 1789. 



Terebratula dilatata, Valenciennes apud Lam., An. sans Vert. vol. vi. p. 215, 1819. 



SECOND SERIES. — ZOOLOGY, VOL. IV. 7 



