14 INTRODUCTION. 



VASCULAR SYSTEM. 



The vascular system of the BracJdopoda is peculiarly well adapted to demonstrate that 

 remarkable condition of the venous system in the Molluscous class which, when first and 

 somewhat imperfectly observed, gave rise to the idea of the blood being extravasated into 

 the lacunae of the viscera, and the interstices of other soft parts and tissues. It was on 

 that account that I was induced to communicate a note, with a few illustrations of the 

 real state of the venous system, as exemplified in the Terebratula and Lingula, to my 

 esteemed and distinguished friend and colleague in the Prench Academy of Sciences, Dr. 

 Milne Edwards, whose descriptions of an analogous condition of the venous system dis- 

 covered by him in other MoUusks might have been interpreted, through the ambiguity of 

 certain terms and expressions, as giving countenance to the theory of extravasation, which 

 had been at that time propounded under the term ' phlebenterism.' In the Number of the 

 ' Annales des Sciences Naturelles' for March 1845, e. g., M. Edwards, in his ' Report on the 

 Results of a Voyage to Sicily/ writes " Chez les Mollusques, meme les plus parfaits, le 

 systeme des vaisseaux a I'aide desquels le sang circule dans I'economie est plus ou moins 

 incomplet ; de sorte que, dans certains points du cercle circulatoire, ce liquide s'epanche dans 

 les grandes cavites du corps ou dans les lacunes dont la substance des tissues est creusee." 

 — p. 139. Mere words rarely turn men from wrong opinions, at least in a science of 

 observation ; but a step, I thought, would be gained in the right direction, if the things, 

 with what might be their more correct interpretation and designations, were placed 

 before the eyes of the accomplished author above cited. The desired result was not, 

 however, attained ; for, in a subsequent number of the ' Annales des Sciences,' May, 

 1845, p. 292, in which my Paper ' On the Circulation of the Brachiopoda' appeared, the 

 able Editor reproduces his former statement in reference to the circulation in the 

 MoUusks ; and, by way of contrast with what he continued to regard as the less correct 

 view, he proceeds to remark: "Effectivement, dans les ouvrages les plus recents sur ces 

 matieres, on dit que cet appareil est un systeme de vaisseaux clos dans lequel le sang de 

 tout le corps est enferme" reference being made to ' Owen, Lectures on Comparative 

 Anatomy and Physiology of the Invertebrate Animals,' p. 13, London, 1843. 



I believe, however, that it is now admitted by most, if not all Malacologists, that a 

 system of arteries and veins is not the less ' a closed and continuous system of circulation,' 

 because the veins exchange their cylindrical for less regular forms, with a concomitant 

 evanescence of all the coats save the innermost tunica propria. 



Such is the condition of the major part of the venous system in the Terebratula. 

 The hearts are two in number, and distinct ; they consist, as in other Brachiopoda, each 

 of an auricle and a ventricle, and are situated, in the Ter.flavescens, at the back part of 

 the visceral cavity, on the dorsal aspect of the intestine, one on each side of its upper or 

 anterior half. 



