CHAPTER VII. 



CIRCULATION IN THE INVEKTEHRATA. 



The circulation of the blood in the liighei' animals was dis- 

 covered by Harvey in 1619. 



In order to nourish all the parts of the body, it is necessary 

 that the blood should be conveyed to these parts ; but the 

 mode in which it is conveyed differs considerably in the 

 lower animals. Among the Invertebrates we find that the 

 mode of circulation becomes more and more specialised as 

 they rise in the zoological scale. From the Protozoa to the 

 Co&lenterata., the circulatory and digestive systems are still 

 fused together, for they are not differentiated. In the 

 Echinodermata and Annelid" we find the first true blood or 

 vascular system. In most worms one of the blood-vessels 

 forms a pulsating tube, or so-called heart, by which the 

 blood is driven towards the periphery of the body through 

 certain vessels, returning by others. In the Mollusca there 

 is a contractile vessel, which has a much closer resemblance 

 to the Vertebrate heart than the above. This heart consists 

 of two or three chambers ; — (r^) One or two auricles, which 

 serve for the reception of the blood, brought to them by the 

 veins ; (/>) the ventricle which serves for the propulsion of 

 the blood into the arteries. It will be noticed from the 

 above remarks that the circulatory system, like all others, 

 was not perfected at once. Nature made numberless 

 attempts, adding successively new pieces to the system, or 

 complicating little by little those which existed already. In 

 other words, the circulatory system became more and more 



