258 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. 



OP 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 



