THE BKACIIIOrODA. 29 



shall term the " atrial ' system, from its close correspondence 

 with the system of cavities which has received the same name 

 in the Ascidians, has been wrongfully 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 "), which 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, by 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 arterial trunks, which 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 well shown in his splendid 

 and exhaustive memoir, published in the Philosophical Trans- 

 actions 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 excre- 

 tory matters and the products of the reproductive organs, which 

 are developed in various parts of the walls of the atrial system. 



The precise characters of the true vascular system of the 

 Brachiopocla probably require still further elaboration than they 

 have yet received ; and the same may be said, notwithstanding 

 the valuable contributions of F. M tiller and of Lacaze Duthiers, 

 of their development ; but the shell, the pallial lobes, the in- 

 testine, and the nervous and the atrial systems, afford characters 

 amply sufficient to define the class. 



The next great division is that of the Ascidioida, which, like 

 the Brachiopoda, are marine animals, and are very common all 

 over the world ; the more ordinary forms of them being always 



