146 BIOLOGY: GENERAL AND MEDICAL 



would be equal to the whole value thus gained. The 

 effete matter is, therefore, a useless encumbrance to 

 the organism, and when derived from nitrogenous com- 

 pounds is injurious if retained, so that we find even the 

 most lowly creatures eliminating or throwing off waste 

 products in some form or other, the process being known 

 as excretion. 



As nearly all of the lowly forms of life are aquatic, 

 their excreta are easily carried away by transfusion. 

 In the corticate protozoa they may be suddenly elim- 

 inated through an anal pore in the ectoderm. 



Primitive metazoan animals whose cells are in two 

 layers, one outer in contact with the water of their 

 habitat, the other, inner, in contact with the water 

 alternately sucked in and forced out of the gastric 

 cavity, as in hydra, or carried through in a continuous 

 stream, as in the porifera, need no special contrivance 

 for the removal of their cellular excrement which is 

 transfused or ejected from the cells. 



It is, therefore, not until structural complexity em- 

 bracing a separation of the blood from the gastric con- 

 tents is reached and the cells become so numerous that 

 many of them are remote from both surface water and 

 watery gastric contents, that some special contrivance 

 by which the cellular waste products can be discharged 

 is required. It is also only at this time that a separation 

 between the waste that results from indigestible rem- 

 nants of food in the gastric cavity and the waste that 

 results from cellular metabolism becomes clear. The 

 former, remaining in the alimentary organs, is discharged 

 through an anal orifice; the latter, collected by the 

 blood, is eliminated through certain lateral pores along 

 the sides of the animal's body. In the description of 

 the circulation of the unsegmented worms it was shown 

 that the contents of the water vascular system in part 

 returns to the digestive organs, but that a small part of 

 it escapes through superficial pores of the skin. This 

 is probably the most primitive form in which excretion 



