216 CIRCULATION IN MOLLUSCA. 



power, and lience we have in these animalcules a circulation 

 in vessels which are alternately arterial and venous,* — a 

 fact, perhaps, that may find its parallel in the minute capil- 

 lary system of the human hody itself. 



I have given you, in the preceding pages, the view of the 

 circulation in mollusca such as I learned it principally from 

 the " Memoires" of Cuvier; but, within these few years, 

 Milne-Edwards has shown that it needs to he qualified, and 

 is, in some respects, erroneous. It is assumed, you will 

 observe, that the blood circulates in closed tubes or vessels 

 forming an uninterrupted system of conduits, — one set of 

 vessels parting from the heart or central reservoir, and divid- 

 ing into branches and smaller branchlets, which gradually 

 lose themselves in the intimate texture of the organs. Here, 

 it is also assumed, that they meet with, and are continued 

 into other ramifications, equally fine, of a set of vessels 

 which, returning from the organs, gradually coalesce into 

 fewer and larger, until they at length constitute only one or 

 a few trunks. There is no break in the continuity of the 

 vessels, — no interruption to the circular current of the 

 stream. Milne-Edwards' extensive researches lead to a dif- 

 ferent conclusion. He finds that in many mollusca, of all 

 orders, a more or less considerable portion of the sangui- 

 ferous circle is always formed by lacunae or interorganic 

 gaps and gutters ; that sometimes, in a considerable part of 

 the body, there are neither arteries -nor veins ; while, in 

 other instances, the arteries distribute the blood throughout 

 the body to every portion where there is life to be supported, 

 but there are no veins to ensure the return of the nutrient 

 fluid, which, on the contrary, is shed into empty spaces 

 between the different solid parts of the organism. Even in 

 those mollusks in which the circulatory apparatus is most 

 complete, and where there are veins as well as arteries, yet 

 do these veins never form a continuous circle, but, on the 

 contrary, the abdominal or peritoneal cavity becomes in 

 part a sanguineous reservoir to interrupt it.* Milne- 

 Edwards thus sums up the result of his numerous anato- 

 mies : — In no mollusk has there been found a closed system 



* Obs. sur les Ascid. Comp. 7 — 9. 



t " Chez les Mollusqucs, de mfeme que chez les Crustaces, line portion plus 

 ou moins considerable du cercle parcouru par le sang en mouvement est tou- 

 jours constituee par les lacunes ou espaces interorganiques ; jamais ce 

 liquide ne se trouve emprisonne, comme on le supposait, dans un systeme 

 clos et complet de vaisseaux a parois propres ; quelquefois il n'existe, pour 

 une portion conside'rable du corps, ni arteres ni veines, d'autres fois les ar- 

 teres portent le sang partout ou il y a vie a entretenir, mais il n'y a pas de 

 veines pour assurer le retour du fluide nourricier qui s'epanche dans les la- 



