220 CIRCULATION IN MOLLUSCA. 



Quatrefages, now known to be erroneous ; but Milne-Ed- 

 wards asserts that the great veins do even in them terminate 

 in a large venous reservoir or lacuna situated in the back, 

 and constituting the great visceral cavity. The Nudibran- 

 chia form, according to him, no exception to his law. In 

 the Thethys and the Eolides the circulation is not only in- 

 complete, but there appear to be few or no proper veins to 

 bring back the blood from the different organs to the bran- 

 chiae, and a system of more or less elongated channels takes 

 their place. The abdominal cavity is a venous reservoir ; 

 and only in the track followed by the arterial blood do we 

 find true vessels, viz., membranous tubes with parietes inde- 

 pendent of the adjacent parts. Should we employ in the 

 description of the mollusca the nomenclature of human 

 anatomy, the enunciation of these results might seem errone- 

 ous, for we should then say that the Thethys, the Eolides, 

 and the Aplysia were provided with veins, for they do pos- 

 sess branchio-cardiac vessels corresponding physiologically 

 to the vessels that carry the arterial blood from the lungs to 

 the heart, and which, in man, are called pulmonary veins ; 

 but as this nomenclature gives a false idea, it is better to 

 limit the name of venous to that portion of the circulating 

 system only which contains the venous blood. The vascular 

 system, then, is incomplete ; branchio-cardiac vessels conduct 

 the arterial blood from the organs of respiration to the heart; 

 arteries, properly so called, distribute the fluid to every part 

 and organ of the body, and thenceforward it is essentially by 

 the media of lacunas or interorganic spaces that the venous 

 blood flows back through the system and gains its goal in 

 the branchiae. 



With much of this the elaborate dissections of Messrs. 

 Hancock and Embleton disagree. They say, " The bran- 

 chio-cardiac sinus figured and described by Milne-Edwards, 

 appears to us to be somewhat anomalous, and certainly differs 

 from anything we have seen either in Eolis or Doris, and is 

 quite at variance with the corresponding part in the Tritoni- 

 adae, of which family it (Thethys) is clearly a member, for in 

 Tritonia hombergii and in Scyllaea pelagica, the auricle is not 

 longitudinally but transversely placed, receiving veins from 

 the skin at each end." In Eolis there is a simple two-cham- 

 bered heart, the blood coming from veins into the auricle, 

 passing then into the ventricle, and being thence propelled 

 along the arteries which carry the blood to the viscera and 

 skin. How the arteries terminate is yet undetermined : " We 

 cannot undertake to say whether they end by closed extre- 

 mities, or whether they have open mouths which communi- 



