126 
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 ascidise 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 cephalopodous"|' 
orders of mollusca have numerous glandular appendages, which appear to absorb 
fluids from the abdomen and pass them to the blood. 
The annelidsej 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 crustacese 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 arachnidse§ the circulation does not differ materially from the crustacese, 
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 bod}% 
i* In which the organs of motion, feet or tentaculse, are supported by the head. 
t Hed-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 Cuvier. 
