THE INNER TISSUES OF ANIMALS. 341 



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- 

 visceral cavity through inosculating sinuses. Among the 

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

 the efferent ones, acquire distinct walls. On putting 



together these facts, we may conceive 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 partially shut off becomes a con- 

 tractile vessel; and by its actions there is produced a more 

 rapid transfer of the nutritive liquid than was originally 

 produced by the motions of the animal. Clearly, the exten- 

 sion of this contractile tube anc^ the development from it of 

 branches running hither and thither into the tissues, must, 

 by defining the channels of blood throughout a part of its 

 course, render its distribution more regular and active. As 

 fast as this centrifugal growth advances, so fast are the effer- 

 ent currents of blood, prevented from escaping laterally, 

 obliged to move from the centre towards the circumference; 

 and so fast also does the less-developed set of channels become, 

 of necessity, occupied by afferent currents. When, by a paral- 

 lel increase of definiteness, the lacunas and irregular sinuses 

 through which the afferent currents pass, become trans- 

 formed into veins, the accompanying disappearance of all 

 stagnant or slow-moving collections of blood, implies a fur- 

 ther improvement in the circulation. 



By what agency is effected this differentiation of a definite 

 vascular system? No sufficient reply is obvious. The 

 genesis of the primordial heart is not comprehensible as a 

 result of direct equilibration, and we cannot readily see our 

 way to it as a result of indirect equilibration ; for it is diffi- 

 cult to imagine what favourable variation natural selection 

 could have seized hold of to produce such a structure. A 

 contractile tube that aided the distribution of nutritive 



