Chap, vi.] LOWER METAZOA. 211 



The cilia of the flagellated chambers of the 

 Sponge (Fig. 53) have clearly the same function 

 among the Porifera, and the currents, which we have 

 already learnt to be food-bearing currents, are the 

 means also by which oxygen is brought to the cells 

 that line the surface of the canals, and, while they 

 bring oxygen, they carry away carbonic acid. Similarly, 

 among the Coeleiiterata,* the necessary oxygen 

 enters in the same way, and passes by the same 

 passages as the food-currents. Among the lower 

 worms, such as the Turbellaria, oxygen is, no 

 doubt, obtained in the same way, but here in all 

 probability the soft ciliated integument also affords a 

 means by which oxygen may be carried to the cells. 



It would be difficult to point to any part of the 

 body of a TVematoid worm which could be reason- 

 ably supposed to have a respiratory function ; on the 

 other hand, such experiments as those of Oerley, who 

 placed a number of specimens of Anguillula aceti (the 

 vinegar paste-worm) in a small vessel and covered 

 them with a layer of oil an inch thick, and found that 

 after two months the greater number were still alive, 

 prove that, in these worms at any rate, there is but a 

 very feeble demand for fresh supplies of oxygen. 



Where a definite blood-vascular system is de- 

 veloped, the consideration of respiratory problems is 

 rendered easier ; for, just as the currents of water in a 

 sponge carry in oxygen as well as food, so does the 

 blood of the higher Metazoa serve as an oxygen 

 carrier as well as a store of food material for the 

 different cells of the body. The oxygenating office is 

 again rendered still more effectual when the blood 



* It has been recently pointed out by Lankester that the so- 

 called SUBGEMTAL PITS of Aurelia have no connection at all with 

 the geni-tal glands. They are spacious cavities, opening to the 

 exterior by comparatively small pores, and they serve, he thinks, 

 as receptacles for respiratory water. They are four in number, 

 and are set close to the mouth, one in each quarter of the disc. 



