THE INNER TISSUES OF ANIMALS. 



331 



stance of the body. This distribution of the plasma, which 

 muscular movement and osmotic distension here combine to 

 aid, is, in somewhat more developed types, further aided by 

 a rudimentary heart : in the peri- visceral sac is seated an 

 open-mouthed tube, along which a wave of contraction pro- 

 ceeds, first for a while in one direction and then again in the 

 opposite direction. The higher orders of Mollusca htive this 

 simple contractile tube developed into a branched 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. 

 Throughout most orders of the Mollusca the back current 

 of blood continues to be carried by channels of the original 

 kind : there are no true veins, but the blood having been 

 delivered into the tissues, finds its way back to the peri- vis- 

 ceral cavity through inosculating sinuses. Among the Ce- 

 phalopods, however, the afferent blood-canals, as well as the 

 efferent ones, acquire distinct walls ; but even here the shut- 

 ting off of the vascular system from the general cavity of the 

 body is not complete; since there are still certain veins which 

 empty themselves into the peri-visceral sac. Put- 



ting together these facts we may see pretty clearly the 

 stages of vascular development. From the original reservoir 

 of nutritive liquid between the alimentary canal and the wall 

 of the body, a portion is partially shut off*; and by the ver- 

 micular contraction of the open tube thus formed, there is 

 i)roduced a more rapid transfer of the nutritive liquid from 

 one part of the peri -visceral sac to another, than was origi- 

 nally produced by the motions of the animal. Clearly, the 

 extension of this contractile tube and the development from 

 it of branches running hither and thither into the tissues, 

 must, by defining the channels of the blood throughout a part 

 of its course, render its distribution more regular and active. 

 As fast as this centrifuoral o^row^th of definite channels advances, 



