134 BIOLOGY: GENERAL AND MEDICAL 



be contractile, a specialization readily observed among 

 the lower worms and laying the foundation of the organ 

 known as the heart. 



The larger vessels are at first in intimate relation with 

 the alimentary tract which they surround with loops. 

 Muscular fibres are present in these large vessels so 

 that as the products of digestion are absorbed into the 

 blood, a slow rhythmical contraction propels the blood 

 in a circuit of the tissues. At first the arrangements are 

 so primitive that the course of the blood is uncertain, 

 but as the specialization becomes improved there is an 

 increasing tendency for the flow to maintain a constant 

 direction, efferent and afferent vessels being differen- 

 tiated and a primitive separation of arteries and veins 

 thus established. In the elementary form in which 

 this condition is observed there are no capillary vessels 

 connecting the two so that the circulation is not closed. 

 True capillaries first appear among certain of the worms, 

 though many higher animals as, for example, insects 

 are without them. In the higher annulates and among 

 the arthropods the major vessel becomes expanded into 

 a primitive heart which receives the blood from several 

 large veins whose orifices are provided with valves pre- 

 venting backward flow so that the stability of the circula- 

 tion is established. The muscular movements of the 

 heart now become rhythmical, regular, and slow. Such 

 simple hearts are found among the arthropoda generally. 



The animals thus far used to exemplify the increasing 

 complexity of the circulatory system are aquatic, of small 

 size, and of simple structure. But as the phylogenetic 

 series is ascended, and increase in complexity is brought 

 about through the differentiation of systems of organs, 

 increase in size, and, eventually, change from the aquatic 

 to the terrestrial mode of life, a new requirement neces- 

 sitates the development of a new system of organs as 

 well as a further increase in the complexity of the al- 

 ready existing organs. This is an increase in the O sup- 

 ply and an improvement in the means by which it is re- 

 ceived and distributed, and is met by the appearance of 



