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If the back of the silk-worm be attentively examined, we shall observe upon it 
a dark coloured line continually in a state of oscillatory movement. This organ is 
analagous to the heart of other animals; and although much discussion has arisen 
among naturalists, with regard to its nature and use, the researches of Carus have 
established beyond doubt, that it is the central organ of the circulation, and both 
sends out and receives fluids. Cuvier considered the dorsal vessel of insects as 
the mere rudiment or vestige of a heart, and supposed that nutrition was effected 
by mere imbibition, as in the lower orders of zoophytes. That a distinct passage 
of fluids from the dorsal vessel does, however, take place, is certain, from the mi¬ 
croscopic researches of Carus, which were first made known to the German natu¬ 
ralists in 1826. The first observations of Carus were made upon the larva of the 
Agrion Puella ;* subsequently upon that of the Ephemera Vulgata, and at length 
upon many insects, both in the larva and imago state. In the first mentioned insect 
which swims with great velocity by means of three vertical laminae attached to the 
caudal extremity of the body, and in which there are at first no traces even of the 
rudiments of wings, Professor Carus found the blood entering by single globules 
from the dorsal vessel into the caudal laminae, passing through them and return¬ 
ing again to the central organ of the circulation. These laminae are composed of 
a granular substance (resembling boiled sago) enveloped by folds of the common 
covering of the body. Into this granular substance the blood passes by single glo¬ 
bules, which are not contained in distinct vessels, but form for themselves a pas¬ 
sage through the homogeneous structure of the body. The path or channel thus 
formed in the midst of the granular substance is perfectly transparent; its sides 
are not strictly defined, nor formed by any thing like the coats of a vessel. This 
extra vascular circulation in the permanent state in insects, is found to exist in the 
embryo state, at the first commencement of organization, in many of the higher 
classes of animals; thus the first appearance of circulation in the incubated egg is 
the movement of a few red globules at points separate from each other, when, 
as yet, no vessels are formed, j- In the aquatic “ ephemera vulgata ,” the circula¬ 
tion is distinctly visible, with the microscope, in the three last segments of the 
body, in the upper phalanges of the legs, in the head, and in the posterior roots of 
the antennae ; it consists, as in the Agrion Puella, of two streams, an excurrent 
and a returning one ; the blood passing through the various parts of the granular 
substance of the body, unconfined in vessels resembling either arteries or veins. 
In 1827, M. Carus discovered the circulation in the fully-developed insect, and 
subsequently Ehrenberg and Hemprich, travellers in Africa, have observed similar 
currents of blood in the wings of a Mantis. 
* A species of Dragon-fly. 
■f For a full and most interesting account of the formation of the ovum in various ani¬ 
mals, and the* development of its several structures, see Breschet’s translation into French 
of the German work of Baer —Sur la Formation de V(Euf. Paris, 1829. 
