

CIRCULATION IN MOLLUSCS. 313 



activity of their movements would necessitate a corresponding rapidity 

 in the circulating current, especially for the purpose of conveying an 

 extraordinary supply of oxygen to the nervous and muscular systems. 

 But this is provided for in another way ; the air being conveyed to 

 these tissues not through the blood, but by direct transmission through 

 the minute ramifications of the air-tubes or tracheae, which penetrate 

 the very smallest organs of the body (,659); 



554. The condition of the Circulating apparatus in the Embryo of 

 higher animals, at a period a little advanced beyond that just alluded to, 

 presents a striking analogy with that last described ; for the heart, at 

 the time of its first formation, seems like a mere dilatation of the princi- 

 pal vascular trunk, having thickened walls, in which, after a time, mus- 

 cular fibre begins to be developed, and the contractile power manifests 

 itself. The pulsation of this heart, however, does not seem to extend 

 its influence immediately through the vascular area ; the capillary circu- 

 lation in which, remains for some time in great degree independent of it. 

 There is no resemblance in form, however, between the dorsal vessel of 

 Insects, and the incipient heart of the higher animals ; since the latter 

 is never much prolonged, and speedily becomes doubled (as it were) 

 upon itself; and its first division into distinct cavities is merely for the 

 purpose of separating its receiving portion, or auricle, from its propelling 

 portion, or ventricle. But the general condition of the Circulating 

 system is much the same in the two cases ; and it is further alike in 

 this, that it is not always easy to show that the vessels have distinct 

 walls, as they frequently seem like mere channels excavated in the 

 tissues. 



555. We may next turn our attention briefly to the condition of the 

 Circulating apparatus in the Molluscous classes, which has lately been 

 found to present some very peculiar characters. In these it would 

 seem as if the moving power were more concentrated in the heart, than 

 in the preceding ; for this organ seems no longer like a mere dilatation 

 of the vascular trunk, but is a distinct sac with muscular walls, usually 

 having at least two cavities, an auricle and a ventricle. The usual 

 course of the Circulation is the following. The blood, expelled from 

 the ventricle of the heart, passes along the main systemic artery, or 

 aorta ; which distributes it to the body at large. It is then collected 

 again, and transmitted to the respiratory organs ; in which it is exposed, 

 either to the air contained in the surrounding water, or (in the terres-. 

 trial Molluscs) more directly to the atmosphere ; and from these it is 

 returned to the heart, to be again transmitted to the system. Thus we 

 see that the heart of these animals receives and impels aerated blood ; 

 and that its office is, to send that blood to the capillaries of the general 

 system. Hence it may be called a systemic heart. 



556. The blood, in the first part of its course, passes through dis- 

 tinct vessels : it has been lately shown, however, that in the Molluscs 

 in general, the blood which has passed through the systemic capillaries, 

 and is on its way to the respiratory organs, is no longer thus confined, 

 but that it meanders through passages or lacuna?^ which are channelled 

 out in the tissues, and which even communicate freely with the abdomi- 

 nal cavity in which the viscera lie; so that their whole exterior is 



