BLOOD OF MOLLUSCA. 5 



the circulation is always double : in other words, they 

 have a systematic circulation through vessels ramifying- 

 over the body, and a pulmonary circulation. By 

 this is meant, a circulation distinct from that of the 

 system^, and permeating the organs of aeration, for the 

 purpose of subjecting the blood to the action of the 

 air, either contained in water, or simply atmospheric. 

 There is, therefore, a heart, consisting either of a 

 single cavity or ventricle, or of more than one, in 

 which case the two cavities do not form one compact 

 organ, as in warm-blooded animals, but are often sepa- 

 rated and at a distance from each other, as if they were 

 distinct hearts. 



Heart of Buccinum undatum ; a, auricle, raised to show tlie ventricle ; b, 

 heart, enlarged, opened to show interior; c, heart and auricle in their cavity. 



The moUusca are usually enveloped in a skin, termed 

 the mantle ; but this mantle differs greatly in its develop- 

 ment and form. The groups in which it is simply 

 membranous or fleshy, are termed naked mollusks ; 



