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In the acephalous* mollusca we have distinct organs of respiration, in the 

 shape of rudimentary gills, and into these organs the veins returning the blood 

 from the body enter. From the gills the blood is sent to the heart, and thence to 

 the body at large, from whence it is again returned to the gills or organs of respi- 

 ration. In the ascidiae there is but one sac, which fulfils the office of a heart ; 

 in the teredo navalis there are four : the whole order of acephalous mollusca pre- 

 sent great varieties in the number of hearts, as also in their form and position. 

 The teredines alone have red blood, probably owing to the excessive stimulus 

 required by the muscles which move the boring shells. In molluscous animals 

 generally, the circulation is double ; the blood passing through the gills, or lungs, 

 or whatever name the respiratory organ may bear, as well as through the body. 

 The passage of the blood through the body is assisted by a muscular heart placed 

 in the course of the blood ; the heart is generally single and aortic, that is, the 

 single ventricle sends the blood to the body, after having received it from the or- 

 gans of respiration : the circulation of fishes, on the contrary, is different, though 

 performed by a single heart, which is termed pulmonary, because it receives the 

 blood from the body and transmits it to the lungs. When the heart is single and 

 sends the blood first to the lungs, it is termed pulmonary ; when the fluid is trans- 

 mitted first to the body, it is termed aortic. The veins in the cephalopodousf 

 orders of mollusca have numerous glandular appendages, which appear to absorb 

 fluids from the abdomen and pass them to the blood. 



The annelidae;): have an arterial and venous circulation, consisting of a central 

 dorsal vessel, which is the artery, and two lateral vessels, which are veins ; there 

 are no hearts placed in the course or centre of the circulation ; the movement of 

 the nutritive fluid is extremely slow, and performed either by the power of its con- 

 taining vessels, or the pressure of the surrounding parts. In these animals the 

 blood is uniformly red but does not exceed in temperature the medium in which 

 they live. In the crustaceae there is an elongated sac upon the dorsal surface of 

 the body, which is the heart ; this organ which receives the blood from the organs 

 of respiration and propels it to the body, approaches in its appearance the dorsal 

 vessel of insects ; it is, in fact, merely a dilated vessel, of which the coats are 

 thicker and more powerful than those of the other parts of the circulating system. 



In the arachnidae§ the circulation does not differ materially from the crustaceae, 

 the function is performed by a dorsal vessel, which is a rudimentary heart sending 

 out blood vessels which are arteries, and receiving others which are veins. 



* Mollusca without apparent head, which is concealed under the mantle in the centre 

 of the body. 



•f- In which the organs of motion, feet or tentaculse, are supported by the head. 



$ Red-blooded worms. 



§ The Spider genus, differing from the Insecta in many particulars, and forming the 

 third class of articulated animals in the Regne Animal, of Cuyier. 



