16 INTRODUCTION. 



small plaits or folds, also radiating from the apex : and the longitudinal or radiating folds are 

 puckered by transverse folds, i, pi. 3, fig. 1 . The figures ii and iii, pi. 3, which are magnified 

 to the same degree, exemplify two of the different states in which the plicated auricles, i, 

 are found in different individuals, and so far exemplify the extent of dilatation and con- 

 traction of which these complex cavities are susceptible ; but they have presented more 

 extreme differences of size in other specimens, and they must possess considerable powers 

 of altering their capacity. It is, therefore, probable, that, when the circulating fluid is 

 accumulated in unusual quantity in the pallial and visceral sinuses, the longitudinal 

 auricular fibres evert and expand the margins of the basal aperture to which the delicate 

 tunic of the sinuses is attached, as shown in fig. 1 , l, 5, 6, that the fluid is drawn by a kind of 

 vermicular movement or peristaltic suction into the auricles, i, i, and is thence propelled by 

 successive contraction of the circular fibres into the ventricles, 2, 2. From the ventricles the 

 blood is driven through the ramifications of the pallial and visceral arteries again into the 

 more or less irregular and capacious sinuses, and so returns slowly back to the heart. 

 In this circulation there is no actual extravasation : I find, after the most patient scrutiny, 

 no evidence of an escape of the blood into mere lacunae or interspaces excavated in the 

 tissues of other and surrounding systems ; but the result of such scrutiny has been, 

 invariably, the detection of the continuation and expansion of the proper tunic of the veins 

 into such seeming lacunse or interspaces. And although, in the wide clefts between the 

 viscera and muscles in the abdominal chamber, the tunic of the venous sinuses is disposed 

 like a peritoneum, seems to perform, also, the function of a peritoneum, and the contained 

 fluid, that of peritoneal serum, in addition to their own more proper and important offices, 

 — and although an anatomist might be permitted, by such similarity of function, to call the 

 cavities of the sinuses 'intervisceral lacunae,' and the walls of the sinuses 'peritoneum,' 

 — yet, if he were guided in his nomenclature by considerations of homology instead of 

 analogy, he would more correctly term them ' abdominal venous sinuses' and ' venous 

 tunic' respectively. In either case, as a matter of fact, there is no real or essential 

 departure from a circulation in a closed system of arteries and veins ; but only a morpho- 

 logical departure from the typical character of the organs of circulation, — an extreme one, it 

 is true, but little likely to lead astray the zootomist who had been prepared for such 

 formal modifications of the vascular system by the discoveries of Hunter, as they are 

 manifested in his preparations, and by the descriptions and figures of such preparations, 

 of the venous system in the classes of Insects and Crustaceans. 



As these clearly enunciated discoveries have been overlooked by the continental 

 Physiologists, who have been stimulated by recent discussions on ' phlebenterism' to take 

 a retrospect of the history of the progress of anatomical research in the field of the 

 vascular system of the Invertebrata,' it may not be without its use, and it is but a mere 

 act of justice to our own great anatomist, to call attention here to his interpretation of 



^ Rapport sur PLI^benterisme, par M. le Dr. Robin, 8vo, 1851. 



