80 SIMPLIFICATION OF STRUCTUBE IN LOWEST ANIMALS. 



substance upon .the supply of blood which it receives, is 

 shown by the fact, that if this supply be temporarily cut off, 

 either by failure of the heart's action (as in fainting), or by 

 pressure on the blood-vessels which convey it, immediate 

 insensibility, with loss of all power of motion, is the result. 

 And the same is the case with regard to the organs of sense ; 

 for if the circulation through them be interrupted, no sensory 

 impression can be made upon the nerve-fibres which originate 

 in them, as we see when the movement of blood in a limb is 

 suspended by pressure upon its artery. 



64. The foregoing constitute the principal tissues among 

 the higher animals, in which the principle of division of labour 

 is most fully carried out, every component part having its 

 own peculiar structure and its own special action. As we de- 

 scend in the scale, we find these distinctions less and less 

 obvious, so that when we come down to Zoophytes ( 121), we 

 meet with but little differentiation either in the textures or in 

 the actions of the several parts of the body ; the whole sub- 

 stance of these animals being composed of a tissue, which 

 very closely resembles that which is first formed in higher 

 animals for the reparation of wounds, having the appearance 

 of a solidified blastema ( 34), with nuclear particles, in 

 various phases of development into cells and fibres, more 

 or less thickly scattered through it ; and this substance 

 being everywhere contractile, and everywhere (at least in 

 many instances) equally capable of participating in the func- 

 tions of nutrition and reproduction. And when we pass still 

 lower, to that simplest type of animal life, which is pre- 

 sented to us in the Rhizopods ( 129), we do not meet with 

 even this amount of definite structure, but find the entire sub- 

 stance of their bodies composed of an apparently homogeneous 

 jelly, which, like the more organized tissue of the Zoophytes, 

 is everywhere contractile, and which has also the power of 

 performing every operation required for its growth and main- 

 tenance as a living being. In such creatures there is not the 

 slightest vestige of a Nervous system ; and it remains a question 

 whether, in consequence of this deficiency, they are altogether 

 destitute of consciousness, or whether this endowment is dif- 

 fused, as it were, through the whole substance of their bodies. 



65. Every component part of the fabric must be regarded 



