2 58 CIRCULATION IN ZOOPHYTES AND SPONGES. 
the polypes, and sometimes from the polypes towards the 
stem ; the rapidity and constancy of these currents depending 
apparently on the activity of the growth of the parts towards 
which they are directed. In the Medusae we find the central 
stomach sending out prolongations towards the margin of the 
disk, where they frequently inosculate so as to form a net¬ 
work, which seems to have for its purpose to expose the 
product of digestion to the aerating action of the surrounding 
water ; and in this system of canals, also, a movement of fluid 
may be observed, which appears to depend upon the action of 
cilia in their interior. In all these cases, it is to be observed 
that the circulation of nutritive fluid is really effected by a 
modification of the digestive apparatus, instead of by an appa¬ 
ratus set apart for this sole purpose ; and the blending of the 
two functions is still more remarkably exhibited in the Sponge, 
the inosculating canals of which (§ 136) may be regarded 
alike as constituting a ramifying digestive cavity, or as a simple 
form of circulating apparatus. The most correct method is 
perhaps to consider it as representing both these systems, 
which are here blended (as it were) into one ; the simplicity 
of structure characteristic of this type not admitting of the 
division of labour which we meet with in higher organisms. 
CHAPTER VI. 
OF RESPIRATION. 
297. We have seen that arterial blood, by its action on the 
living tissues, loses those qualities which rendered it fit for 
the maintenance of life; and that after having undergone 
this change, it recovers its original properties by exposure to 
air. This exposure is necessary, therefore, to the continued 
existence of Animals in general. If we place an animal 
under the receiver of an air-pump, and exhaust the air either 
partially or completely, a great disturbance soon shows itself 
in its various functions; shortly afterwards, the several 
actions of life cease to take place; and a state of apparent 
death comes on, which speedily becomes real, if air be not 
re-admitted. The influence of air is not less necessary to 
