34 An Introihct'ion to Zoology. 



been, as it were, educed out of each other by a development successively 

 progressive ; and indeed we shall find not at all a single linear series, but 

 rather different parallel series, each exhibiting a correspondence of ultimate 

 results, although the intermediate steps of each are widely dissimilar. 



Thus in the insects, beings whose organs of sense are very far advanced, 

 and who evince instincts very highly developed, we have traced from its 

 first imperfect rudiments, a system of vascular circulation, witli a heart as 

 its motive power, and regular organs of respiration. But if we take a step, 

 which, to every mind not prejudiced by the shackles of artificial system, 

 must seem a very great descent, to tlie bivalve mollusca ; the oyster for 

 instance, affixed for life to a single spot, destitute of every organ of sense, 

 and manifesting no higher instinct than that which directs it to open its 

 mouth for the reception of its food; we sliall find, in this low animal, the 

 whole system of circulation and respiration far more analogous to that of 

 the higher orders, than they are in the ant or the bee j and yet these are 

 beings upon whose social and moral history, as it may justly be called, 

 there is room for volumes. 



In the acephalous, or headless bivalve mollusca, we find the gills or 

 branchiae ranged in four lamelliform fringes round the margin of the body, 

 the mouth opening on one side under the hinge, and the vent on the 

 other, the veins collecting the blood from the system generally, and the 

 nutritise chyle from tlie intestines, conveying the mingled stream to the 

 heart, from whence it is again to be disseminated by the arteries through 

 the system. 



Among many of the univalve mollusca, higher and more advanced ani- 

 mals, a similar arrangement is found to prevail. These animals possess a 

 distinct head, provided with organs of vision ; but many of them do not 

 breathe water by gills, but air by a regular pulmonary cavity or apparatus 

 of lungs, resembling in every thing but the number of the cavities, here only 

 one, the organ described for a similar purpose in the arachnida. 



Looking back then upon these classes of animals, we see nothing like a 

 regular progression from aquatic to aerial respiration ; but the most nearly 

 allied genera of the same family are provided with one or the other as 

 accommodated to their intended stations, exhibiting every where the direct 

 intervention of an In'eliigent appointment and design, varying its means 

 according to a varying end, and displaying nothing like the constant and 

 invariable sequence which marks the blind operation of merely physical 

 causes. 



In the ccphalopodes — the cuttle-fish family — are found three hearts; two 

 branchial sinuses, receiving the blood from the venae cavte, and transferring 

 it to the branchiaj, whence it proceeds by the branchial veins to a third or 

 central heart, by which it is diffused through the aorta over the system — 

 an organisation very analogous to that of the Crustacea. The cephalopoda 

 possess organs of vision as highly organised as those of the fishes. In 



