BEEATHING. 61 



the tube. In the second place the ciliated funnels are believed to take up 

 solid waste particles floating in the coelomic fluid and to pass them on into 

 the tube, whence they are ultimately voided to the exterior together with 

 the liquid products described above. It is nearly certain that these parti- 

 cles are derived from the breaking up of " lymphoid " cells, some of which 

 may have been phagocytes (p. 53), floating in the coelomic fluid, and that 

 most if not all of these cells arise from " chloragogue cells " set free from 

 the surface of the blood-vessels and of the intestine. 



Respiration. Respiration, or breathing, is a twofold operation, 

 consisting of the taking in of free oxygen and the giving off of 

 carbon dioxide by gaseous diffusion through the surface of the 

 body. Strictly speaking, this free oxygen must be regarded as 

 food, while carbon dioxide is to be regarded as one of the excre- 

 tions. Hence respiration is tributary both to alimentation and to 

 excretion ; but since many animals possess special mechanisms to 

 carry on respiration, it is convenient and customary to treat of 

 it as a distinct process. 



Respiration is essentially an exchange of gases between the 

 blood and the air, carried on through a delicate membrane lying 

 between them. The earthworm represents the simplest condi- 

 tions possible, since the exchange takes place all over the body, 

 precisely as in a plant. Its moist and delicate walls are every- 

 where traversed by a line network of blood- vessels lying just 

 beneath the surface. The oxygen of the air, either in the 

 atmosphere or dissolved in water, readily diffuses into the blood 

 at all points, and carbon dioxide makes its exit in the reverse 

 direction. Freed of carbon dioxide and enriched with oxygen, 

 the blood is then carried away by the circulation to the inner 

 parts, where it gives up its oxygen to the tissues and becomes 

 once more laden with carbon dioxide. 



In higher animals it has been proved that the red coloring 

 matter (haemoglobin) is the especial vehicle for the absorption 

 and carriage of the oxygen of the blood, entering into a loose 

 chemical union with it and readily setting it free again under the 

 appropriate conditions. This is doubtless true in the earthworm 

 also. 



It is interesting to study the various devices by which this function is 

 performed in different animals. In the earthworm the whole outer surface 

 is respiratory, and no special respiratory organs exist. In other animals 

 such organs arise simply by the differentiation of certain regions of the 



