340 PHYSIOLOGICAL DEVELOPMENT. 





movement of the nutritive liquids which the functions set 

 up. How the second cause — the changes of internal 



pressure which an animal's movements produce — furthers 

 circulation, will be sufficiently manifest. That parts which 

 are bent or strained necessarily have their contained vessels 

 squeezed, has been shown (§ 281) ; and whether the bend or 

 strain is caused, as in a plant, by an external force, or, as 

 usually in an animal, by an internal force, there must be a 

 thrusting of liquids towards places of least resistance — com- 

 monly places of greatest consumption. This which in animals 

 without hearts is a main agent of circulation, continues to 

 further it very considerably even among the highest animals. 

 In these the effect becomes as it were systematized. The valve 

 in the veins necessitate perpetual propulsions towards the 

 heart. 



Even in such simple types as the Hydrozoa, cavities in th* 

 tissues faintly indicate a structure which facilitates the trans- 

 fer of nutritive matters. These cavities become reservoirs 

 filled with the plasma that slowly oozes through the substance 

 of the body ; and every movement of the animal, accompanied 

 as it must be by changed pressures and tensions on these 

 reservoirs, tends here to fill them and there to squeeze out 

 their contents in that or the other direction — possibly aiding 

 to produce, by union of several cavities, those lacunas or 

 irregular canals which the body in some cases presents. 



Irregular canals of this kind, not lined with any mem- 

 branes but being simply cavities running through the flesh, 

 mainly constitute the vascular system in Polyzoa and Brach- 

 iopoda and some Mollusca. Though the central parts of a 

 Vascular system are rudely developed, yet its peripheral parts 

 consist of sinuses permeating the tissues. The higher orders 

 of Mollusca have a more developed system of vessels or 

 arteries, which run into the substance of the body and end 

 in lacunae or simple fissures. This ending in lacunae takes 

 place at various distances from the vascular centre. In some 

 genera the arterial structure is carried to the periphery of 

 the blood-system, while in others it stops short midway. 



