$0 SIMPLIFICATION OF STRUCTUBE IN LOWEST ANIMALS. 
substance upon tbe 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 Bhizopods (§ 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 s}^stem; 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 
