32 ^n Introduction to Zoology. 



that here the blood is not, as in other animals, necessitated to go in search 

 of the air, since the air comes in search of the blood. 



The system of circulation then, begins in insects, and this is so intimately 

 connected with the conveyance of blood to the organs of its aeration, that 

 we must bestow a few words upon that subject. 



This system requires that the vessels conveying the fluid should be fur- 

 nished with a series of valves, favouring its passage in one direction, and 

 impeding or opposing it in the contrary ; and that to such a vascular ar- 

 rangement should be superadded a muscular power, which by a series of 

 alternate pulsations of contraction and dilation — a systole and diastole — 

 should create and preserve an impulse of motion to the sanguinic currents. 

 Such a power may of course reside in the whole length of a continuous 

 vessel, or it may be concentrated upon a circumscribed portion of it ; one 

 or more sinuses for instance of a muscular structure opening into the prin- 

 cipal vessels ; or the power of contracting may reside in both. The 

 muscular organ in the second of these cases is called a heart. A heart, 

 speaking generally, is composed of two sinuses, by one of which, the 

 auricle, it receives the "blood from the system by the main vein ; and by the 

 other, the ventricle, it proi)els the received blood into the system, through 

 the main artery, the aorta. In the higher animals, the mammalia for in- 

 stance, this organ is always double, possessing two auricles and two ven- 

 tricles. Insects have no heart, properly so called. In place of it they 

 have a long contractile and pulsating trunk extending along the back, and 

 thence called the dorsal vessel. This vessel, by a happy arrangement of its 

 fibres, propels the blood from its anterior, and imbibes it by its posterior 

 extremity; the blood thus impelled returning from the former to the latter 

 in its course through the general pores of the system. Such an arrangement 

 is called a "diffused circulation," in contradistinction from that which is 

 carried on by perhaps more circumscribed channels and conduits of distinct 

 vessels. Besides this general diffused circulation, however, which conveys 

 the blood in rapid motion between the numerous ramifications of the tra- 

 chsea, for the purposes of aeration, insects have also lateral blood vessels, 

 sending out loops to the wings, antennae, &c. ; and uniting with the opposite 

 extremities of the dorsal vessel, so as to form distinct channels participating 

 in the same current vith the generally diffused circulation. We have not 

 as yet, however, any aorta or main arterial trunk issuing from the ven- 

 tricular division of the heart, and thus successively ramifying into thousands 

 of smaller branches, conveying blood even to the minutest portions of the 

 body, where they meet extremities of the veins which imbibe the vital 

 streams and carry them backwards, conveying and becoming confluent into 

 larger trunks which at length unite in the grand venae cavae, by which the 

 blood is restored to the heart through its recipient ventricle. For such is 

 the higiier system of circulation to which we are gradually approaching. 



The arachnida, who form the next step to be ascended, possess connected 



